Compare ECMAScript features between the ECMAScript 2021 (ES12) and ES5 Standards JavaScript modes in Washington DC. Both modes support a subset of ECMAScript features.

For more information about these features, see the ECMAScript language specifications (ECMA-262) on the Ecma International website.

Support definitions

Supported
The feature has been tested and validated.
Not Supported
The feature has not been validated in the current release.
Disallowed
The feature does not align with the Now Platform programming model or poses a security or performance risk. Disallowed features result in an error.

ECMAScript 2022 (ES13) features

Important: Prior to deploying code to production, you should test scripts using supported ECMAScript 2022 (ES13) features thoroughly due to the newly added and partial support of features across this ECMAScript version.
Table 1. Instance class fields
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
public instance class fields Supported Not Supported
private instance class fields basic support Not Supported Not Supported
private instance class fields initializers Not Supported Not Supported
optional private instance class fields access Not Supported Not Supported
optional deep private instance class fields access Not Supported Not Supported
computed instance class fields Supported Not Supported
Table 2. Static class fields
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
public static class fields Supported Not Supported
static class fields use [[Define]] Supported Not Supported
private static class fields Supported Not Supported
computed static class fields Supported Not Supported
Table 3. Private class methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
private instance methods Not Supported Not Supported
private static methods Supported Not Supported
private accessor properties Not Supported Not Supported
private static accessor properties Supported Not Supported
Table 4. .at() method on the built-in indexables
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Array.prototype.at() Not Supported Not Supported
String.prototype.at() Supported Not Supported
%TypedArray%.prototype.at() Disallowed Disallowed
Table 5. Object.hasOwn
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Basic functionality Supported Not Supported
ToObject called before ToPropertyKey Supported Not Supported
Table 6. Error.cause property
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Error has cause Supported Not Supported
Error.prototype lacks cause Supported Not Supported
EvalError has cause Supported Not Supported
EvalError.prototype lacks cause Supported Not Supported
RangeError has cause Supported Not Supported
RangeError.prototype lacks cause Supported Not Supported
ReferenceError has cause Supported Not Supported
ReferenceError.prototype lacks cause Supported Not Supported
SyntaxError has cause Supported Not Supported
SyntaxError.prototype lacks cause Supported Not Supported
TypeError has cause Supported Not Supported
TypeError.prototype lacks cause Supported Not Supported
URIError has cause Supported Not Supported
URIError.prototype lacks cause Supported Not Supported
AggregateError has cause Supported Not Supported
AggregateError.prototype lacks cause Supported Not Supported
Table 7. RegExp Match Indices (`hasIndices` / `d` flag)
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
constructor supports it Not Supported Not Supported
shows up in flags Not Supported Not Supported
Table 8. Ergonomic brand checks for private fields
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Ergonomic brand checks for private fields Not Supported Not Supported
Table 9. Class static initialization blocks
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Class static initialization blocks Supported Not Supported

ECMAScript 2021 (ES12) features

Table 10. Promise.any
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
fulfillment Disallowed Disallowed
AggregateError Disallowed Disallowed
Table 11. WeakReferences
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
WeakRef minimal support Disallowed Disallowed
FinalizationRegistry minimal support Disallowed Disallowed
Table 12. Logical assignment
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
||= basic support Supported Not Supported
||= short-circuiting behavior Supported Not Supported
||= setter not unnecessarily invoked Supported Not Supported
&&= basic support Supported Not Supported
&&= short-circuiting behavior Supported Not Supported
&&= setter not unnecessarily invoked Supported Not Supported
??= basic support Supported Not Supported
??= short-circuiting behavior Supported Not Supported
??= setter not unnecessarily invoked Supported Not Supported
Table 13. Numeric separators
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
numeric separators Supported Not Supported
Table 14. String.prototype.replaceAll
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
String.prototype.replaceAll Supported Supported

ECMAScript 2020 (ES11) features

Table 15. String.prototype.matchAll
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic functionality Supported Not Supported
throws on non-global regex Supported Not Supported
Table 16. BigInt
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Not Supported Not Supported
constructor Not Supported Not Supported
BigInt.asUintN Not Supported Not Supported
BigInt.asIntN Not Supported Not Supported
BigInt64Array Not Supported Not Supported
BigUint64Array Not Supported Not Supported
DataView.prototype.getBigInt64 Not Supported Not Supported
DataView.prototype.getBigUint64 Not Supported Not Supported
Table 17. globalThis
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
"globalThis" global property is global object Disallowed Disallowed
"globalThis" global property has correct property descriptor Disallowed Disallowed
Table 18. Optional chaining operator (?.)
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
optional property access Supported Not Supported
optional bracket access Supported Not Supported
optional method call Supported Not Supported
optional function call Supported Not Supported
spread parameters after optional chaining Supported Not Supported
Table 19. Promise.allSettled
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Promise.allSettled Disallowed Disallowed
Table 20. Nullish coalescing operator (??)
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
nullish coalescing operator (??) Supported Not Supported

ECMAScript 2019 (ES10) features

Table 21. Symbol.prototype.description
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic Supported Not Supported
empty description Supported Not Supported
undefined description Supported Not Supported
Table 22. String trimming
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
String.prototype.trimLeft Supported Supported
String.prototype.trimRight Supported Supported
String.prototype.trimStart Supported Not Supported
String.prototype.trimEnd Supported Not Supported
Table 23. Array.prototype.{flat, flatMap}
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Array.prototype.flat Supported Not Supported
Array.prototype.flatMap Supported Not Supported
flat and flatMap in Array.prototype[@@unscopables] Supported Not Supported
Table 24. Object.fromEntries
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Object.fromEntries Supported Not Supported
Table 25. Optional catch binding
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic Disallowed Disallowed
await Disallowed Disallowed
yield Disallowed Disallowed
Table 26. Function.prototype.toString revision
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
functions created with the Function constructor Disallowed Disallowed
arrows Disallowed Disallowed
[native code] Disallowed Disallowed
class expression with implicit constructor Disallowed Disallowed
class expression with explicit constructor Disallowed Disallowed
unicode escape sequences in identifiers Disallowed Disallowed
methods and computed property names Disallowed Disallowed
Table 27. JSON superset
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Line separator can appear in string literals Disallowed Disallowed
Paragraph separator can appear in string literals Disallowed Disallowed
Table 28. Well-formed JSON.stringify
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Well-formed JSON.stringify Disallowed Disallowed

ECMAScript 2018 (ES9) features

Table 29. Object rest/spread properties
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
object rest properties Supported Not Supported
object spread properties Supported Not Supported
Table 30. Promise.prototype.finally
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic support Disallowed Disallowed
don't change resolution value Disallowed Disallowed
change rejection value Disallowed Disallowed
Table 31. Asynchronous iterators
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
async generators Disallowed Disallowed
for-await-of loops Disallowed Disallowed
Table 32. s (dotAll) flag for regular expressions
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
s (dotAll) flag for regular expressions Supported Not Supported
Table 33. RegExp named capture groups
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
RegExp named capture groups Supported Not Supported
Table 34. RegExp Lookbehind Assertions
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
RegExp Lookbehind Assertions Not Supported Not Supported
Table 35. RegExp Unicode Property Escapes
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
RegExp Unicode Property Escapes Not Supported Not Supported
Table 36. Template literal revision
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
template literal revision Disallowed Disallowed

ECMAScript 2017 (ES8) features

Table 37. Object static methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.values Supported Not Supported
Object.entries Supported Not Supported
Object.getOwnPropertyDescriptors Supported Not Supported
Object.getOwnPropertyDescriptors doesn't provide undefined descriptors Not Supported Not Supported
Table 38. String padding
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
String.prototype.padStart Supported Not Supported
String.prototype.padEnd Supported Not Supported
Table 39. Trailing commas in function syntax
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
in parameter lists Supported Not Supported
in argument lists Supported Not Supported
Table 40. Async functions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
return Disallowed Disallowed
throw Disallowed Disallowed
no line break between async and function Disallowed Disallowed
no "prototype" property Disallowed Disallowed
await Disallowed Disallowed
await, rejection Disallowed Disallowed
must await a value Disallowed Disallowed
can await non-Promise values Disallowed Disallowed
cannot await in parameters Disallowed Disallowed
async methods, object literals Disallowed Disallowed
async methods, classes Disallowed Disallowed
async arrow functions in methods, classes Disallowed Disallowed
async arrow functions Disallowed Disallowed
correct prototype chain Disallowed Disallowed
async function prototype, Symbol.toStringTag Disallowed Disallowed
async function constructor Disallowed Disallowed
Table 41. Shared memory and atomics
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
SharedArrayBuffer Disallowed Disallowed
SharedArrayBuffer[Symbol.species] Disallowed Disallowed
SharedArrayBuffer.prototype.byteLength Disallowed Disallowed
SharedArrayBuffer.prototype.slice Disallowed Disallowed
SharedArrayBuffer.prototype[Symbol.toStringTag] Disallowed Disallowed
Atomics.add Disallowed Disallowed
Atomics.and Disallowed Disallowed
Atomics.compareExchange Disallowed Disallowed
Atomics.exchange Disallowed Disallowed
Atomics.wait Disallowed Disallowed
Atomics.wake Disallowed Disallowed
Atomics.isLockFree Disallowed Disallowed
Atomics.load Disallowed Disallowed
Atomics.or Disallowed Disallowed
Atomics.store Disallowed Disallowed
Atomics.sub Disallowed Disallowed
Atomics.xor Disallowed Disallowed
Table 42. Object.prototype getter/setter methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
__defineGetter__ Disallowed Disallowed
__defineGetter__, symbols Disallowed Disallowed
__defineGetter__, ToObject(this) Disallowed Disallowed
__defineSetter__ Disallowed Disallowed
__defineSetter__, symbols Disallowed Disallowed
__defineSetter__, ToObject(this) Disallowed Disallowed
__lookupGetter__ Disallowed Disallowed
__lookupGetter__, prototype chain Disallowed Disallowed
__lookupGetter__, symbols Disallowed Disallowed
__lookupGetter__, ToObject(this) Disallowed Disallowed
__lookupGetter__, data properties can shadow accessors Disallowed Disallowed
__lookupSetter__ Disallowed Disallowed
__lookupSetter__, prototype chain Disallowed Disallowed
__lookupSetter__, symbols Disallowed Disallowed
__lookupSetter__, ToObject(this) Disallowed Disallowed
__lookupSetter__, data properties can shadow accessors Disallowed Disallowed
Table 43. Proxy internal calls, getter/setter methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
__defineGetter__ Disallowed Disallowed
__defineSetter__ Disallowed Disallowed
__lookupGetter__ Disallowed Disallowed
__lookupSetter__ Disallowed Disallowed

ECMAScript 2016 (ES7) features

Table 44. Exponentiation (**) operator
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic support Supported Not Supported
assignment Supported Not Supported
early syntax error for unary negation without parentheses Disallowed Disallowed
Table 45. Array.prototype.includes
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Array.prototype.includes Supported Not Supported
Array.prototype.includes is generic Not Supported Not Supported
%TypedArray%.prototype.includes Disallowed Disallowed

ECMAScript 2015 (ES6) features

Table 46. Proper tail calls (tail call optimization)
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
direct recursion Disallowed Disallowed
mutual recursion Disallowed Disallowed
Table 47. Default function parameters
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
explicit undefined defers to the default Supported Not Supported
defaults can refer to previous parameters Supported Not Supported
arguments object interaction Supported Not Supported
temporal dead zone Disallowed Disallowed
separate scope Supported Not Supported
new Function() support Disallowed Disallowed
Table 48. Rest parameters
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
function 'length' property Supported Not Supported
arguments object interaction Not Supported Not Supported
can't be used in setters Disallowed Disallowed
new Function() support Disallowed Disallowed
Table 49. Spread syntax for iterable objects
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
with arrays, in function calls Supported Not Supported
with arrays, in array literals Supported Not Supported
with sparse arrays, in function calls Supported Not Supported
with sparse arrays, in array literals Supported Not Supported
with strings, in function calls Supported Not Supported
with strings, in array literals Supported Not Supported
with astral plane strings, in function calls Supported Not Supported
with astral plane strings, in array literals Supported Not Supported
with generator instances, in calls Disallowed Disallowed
with generator instances, in arrays Disallowed Disallowed
with generic iterables, in calls Supported Not Supported
with generic iterables, in arrays Supported Not Supported
with instances of iterables, in calls Supported Not Supported
with instances of iterables, in arrays Supported Not Supported
spreading non-iterables is a runtime error Supported Not Supported
Table 50. Object literal extensions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
computed properties Supported Not Supported
shorthand properties Supported Not Supported
shorthand methods Supported Not Supported
string-keyed shorthand methods Supported Not Supported
computed shorthand methods Supported Not Supported
computed accessors Supported Not Supported
Table 51. For-of loops
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
with arrays Supported Not Supported
with sparse arrays Supported Not Supported
with strings Supported Not Supported
with astral plane strings Supported Not Supported
with generator instances Disallowed Disallowed
with generic iterables Supported Not Supported
with instances of generic iterables Supported Not Supported
iterator closing, break Supported Not Supported
iterator closing, throw Supported Not Supported
Table 52. Octal and binary literals
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
octal literals Supported Not Supported
binary literals Supported Not Supported
octal supported by Number() Not Supported Not Supported
binary supported by Number() Not Supported Not Supported
Table 53. Template literals
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
toString conversion Supported Not Supported
tagged template literals Supported Not Supported
passed array is frozen Supported Not Supported
line break normalization Disallowed Disallowed
TemplateStrings call site caching Supported Not Supported
TemplateStrings permanent caching Supported Not Supported
Table 54. RegExp "y" and "u" flags
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
"y" flag Supported Not Supported
"y" flag, lastIndex Supported Not Supported
"u" flag Not Supported Not Supported
"u" flag, non-BMP Unicode characters Not Supported Not Supported
"u" flag, Unicode code point escapes Not Supported Not Supported
"u" flag, case folding Not Supported Not Supported
Table 55. Destructuring, declarations
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
with arrays Supported Not Supported
with sparse arrays Supported Not Supported
with strings Supported Not Supported
with astral plane strings Supported Not Supported
with generator instances Disallowed Disallowed
with generic iterables Supported Not Supported
with instances of generic iterables Supported Not Supported
iterator closing Supported Not Supported
trailing commas in iterable patterns Supported Not Supported
with objects Supported Not Supported
object destructuring with primitives Supported Not Supported
trailing commas in object patterns Supported Not Supported
throws on null and undefined Supported Not Supported
computed properties Supported Not Supported
multiples in a single var statement Supported Not Supported
nested Supported Not Supported
in for-in loop heads Supported Not Supported
in for-of loop heads Supported Not Supported
in catch heads Supported Not Supported
rest Supported Not Supported
defaults Supported Not Supported
defaults, let temporal dead zone Disallowed Disallowed
Table 56. Destructuring, assignment
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
with arrays Supported Not Supported
with sparse arrays Supported Not Supported
with strings Supported Not Supported
with astral plane strings Supported Not Supported
with generator instances Disallowed Disallowed
with generic iterables Supported Not Supported
with instances of generic iterables Supported Not Supported
iterator closing Supported Not Supported
iterable destructuring expression Supported Not Supported
chained iterable destructuring Supported Not Supported
trailing commas in iterable patterns Supported Not Supported
with objects Supported Not Supported
object destructuring with primitives Supported Not Supported
trailing commas in object patterns Supported Not Supported
object destructuring expression Supported Not Supported
parenthesized left-hand-side is a syntax error Disallowed Disallowed
chained object destructuring Supported Not Supported
throws on null and undefined Supported Not Supported
computed properties Supported Not Supported
nested Supported Not Supported
rest Supported Not Supported
nested rest Supported Not Supported
empty patterns Supported Not Supported
defaults Supported Not Supported
Table 57. Destructuring, parameters
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
with arrays Supported Not Supported
with sparse arrays Supported Not Supported
with strings Supported Not Supported
with astral plane strings Supported Not Supported
with generator instances Disallowed Disallowed
with generic iterables Supported Not Supported
with instances of generic iterables Supported Not Supported
iterator closing Supported Not Supported
trailing commas in iterable patterns Supported Not Supported
with objects Supported Not Supported
object destructuring with primitives Supported Not Supported
trailing commas in object patterns Supported Not Supported
throws on null and undefined Supported Not Supported
computed properties Supported Not Supported
nested Supported Not Supported
'arguments' interaction Supported Not Supported
new Function() support Disallowed Disallowed
in parameters, function 'length' property Supported Not Supported
rest Supported Not Supported
empty patterns Supported Not Supported
defaults Supported Not Supported
defaults, separate scope Supported Not Supported
defaults, new Function() support Disallowed Disallowed
aliased defaults, arrow function Supported Not Supported
shorthand defaults, arrow function Supported Not Supported
duplicate identifier Disallowed Disallowed
Table 58. Unicode code point escapes
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
in strings Supported Not Supported
in identifiers Not Supported Not Supported
in property key definitions Not Supported Not Supported
in property key accesses Not Supported Not Supported
Table 59. New.target
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
in constructors Not Supported Not Supported
assignment is an early error Disallowed Disallowed
Table 60. Const
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic support Supported Supported
is block-scoped Supported Not Supported
scope shadow resolution Supported Not Supported
cannot be in statements Disallowed Disallowed
redefining a const is an error Disallowed Disallowed
for loop statement scope Supported Not Supported
for-in loop iteration scope Supported Not Supported
for-of loop iteration scope Supported Not Supported
temporal dead zone Not Supported Not Supported
basic support (strict mode) Supported Supported
is block-scoped (strict mode) Supported Not Supported
scope shadow resolution (strict mode) Supported Not Supported
cannot be in statements (strict mode) Disallowed Disallowed
redefining a const (strict mode) Disallowed Disallowed
for loop statement scope (strict mode) Supported Not Supported
for-in loop iteration scope (strict mode) Supported Not Supported
for-of loop iteration scope (strict mode) Supported Not Supported
temporal dead zone (strict mode) Not Supported Not Supported
Table 61. Let
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic support Supported Not Supported
is block-scoped Supported Not Supported
scope shadow resolution Supported Not Supported
cannot be in statements Disallowed Disallowed
for loop statement scope Supported Not Supported
temporal dead zone Not Supported Not Supported
for/for-in loop iteration scope Supported Not Supported
for-in loop binding shadowing parameter Disallowed Disallowed
basic support (strict mode) Supported Not Supported
is block-scoped (strict mode) Supported Not Supported
scope shadow resolution (strict mode) Supported Not Supported
cannot be in statements (strict mode) Disallowed Disallowed
for loop statement scope (strict mode) Supported Not Supported
temporal dead zone (strict mode) Not Supported Not Supported
for/for-in loop iteration scope (strict mode) Supported Not Supported
for-in loop binding shadowing parameter (strict mode) Disallowed Disallowed
Table 62. Block-level function declaration
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
block-level function declaration Supported Not Supported
Table 63. Arrow functions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
0 parameters Supported Not Supported
1 parameter, no brackets Supported Not Supported
multiple parameters Supported Not Supported
lexical "this" binding Supported Not Supported
"this" unchanged by call or apply Supported Not Supported
can't be bound, can be curried Supported Not Supported
lexical "arguments" binding Supported Not Supported
no line break between parameters and => Disallowed Disallowed
correct precedence Disallowed Disallowed
no "prototype" property Not Supported Not Supported
lexical "super" binding in constructors Supported Not Supported
lexical "super" binding in methods Supported Not Supported
lexical "new.target" binding Not Supported Not Supported
Table 64. Class
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
class statement Supported Not Supported
is block-scoped Supported Not Supported
class expression Supported Not Supported
anonymous class Supported Not Supported
constructor Supported Not Supported
prototype methods Supported Not Supported
string-keyed methods Supported Not Supported
computed prototype methods Supported Not Supported
optional semicolons Supported Not Supported
static methods Supported Not Supported
computed static methods Supported Not Supported
accessor properties Supported Not Supported
computed accessor properties Supported Not Supported
static accessor properties Supported Not Supported
computed static accessor properties Supported Not Supported
class name is lexically scoped Supported Not Supported
computed names, temporal dead zone Not Supported Not Supported
methods aren't enumerable Supported Not Supported
implicit strict mode Not Supported Not Supported
constructor requires new Supported Not Supported
extends Supported Not Supported
extends expressions Supported Not Supported
extends null Supported Not Supported
new.target Supported Not Supported
Table 65. Super
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
statement in constructors Supported Not Supported
expression in constructors Supported Not Supported
in methods, property access Supported Not Supported
in methods, method calls Supported Not Supported
method calls use correct "this" binding Supported Not Supported
constructor calls use correct "new.target" binding Supported Not Supported
is statically bound Supported Not Supported
super() invokes the correct constructor Supported Not Supported
Table 66. Generators
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
generator function expressions Disallowed Disallowed
correct "this" binding Disallowed Disallowed
can't use "this" with new Disallowed Disallowed
sending Disallowed Disallowed
%GeneratorPrototype% Disallowed Disallowed
%GeneratorPrototype% prototype chain Disallowed Disallowed
%GeneratorPrototype%.constructor Disallowed Disallowed
%GeneratorPrototype%.throw Disallowed Disallowed
%GeneratorPrototype%.return Disallowed Disallowed
yield operator precedence Disallowed Disallowed
yield *, arrays Disallowed Disallowed
yield *, sparse arrays Disallowed Disallowed
yield *, strings Disallowed Disallowed
yield *, astral plane strings Disallowed Disallowed
yield *, generator instances Disallowed Disallowed
yield *, generic iterables Disallowed Disallowed
yield *, instances of iterables Disallowed Disallowed
yield * on non-iterables is a runtime error Disallowed Disallowed
yield *, iterator closing Disallowed Disallowed
yield *, iterator closing via throw() Disallowed Disallowed
shorthand generator methods Disallowed Disallowed
string-keyed shorthand generator methods Disallowed Disallowed
computed shorthand generators Disallowed Disallowed
shorthand generator methods, classes Disallowed Disallowed
computed shorthand generators, classes Disallowed Disallowed
shorthand generators can't be constructors Disallowed Disallowed
Table 67. Typed arrays
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Int8Array Disallowed Disallowed
Uint8Array Disallowed Disallowed
Uint8ClampedArray Disallowed Disallowed
Int16Array Disallowed Disallowed
Uint16Array Disallowed Disallowed
Int32Array Disallowed Disallowed
Uint32Array Disallowed Disallowed
Float32Array Disallowed Disallowed
Float64Array Disallowed Disallowed
DataView (Int8) Disallowed Disallowed
DataView (Uint8) Disallowed Disallowed
DataView (Int16) Disallowed Disallowed
DataView (Uint16) Disallowed Disallowed
DataView (Int32) Disallowed Disallowed
DataView (Uint32) Disallowed Disallowed
DataView (Float32) Disallowed Disallowed
DataView (Float64) Disallowed Disallowed
ArrayBuffer[Symbol.species] Disallowed Disallowed
constructors require new Disallowed Disallowed
constructors accept generic iterables Disallowed Disallowed
correct prototype chains Disallowed Disallowed
%TypedArray%.from Disallowed Disallowed
%TypedArray%.of Disallowed Disallowed
%TypedArray%.prototype.subarray Disallowed Disallowed
%TypedArray%.prototype.join Disallowed Disallowed
%TypedArray%.prototype.indexOf Disallowed Disallowed
%TypedArray%.prototype.lastIndexOf Disallowed Disallowed
%TypedArray%.prototype.slice Disallowed Disallowed
%TypedArray%.prototype.every Disallowed Disallowed
%TypedArray%.prototype.filter Disallowed Disallowed
%TypedArray%.prototype.forEach Disallowed Disallowed
%TypedArray%.prototype.map Disallowed Disallowed
%TypedArray%.prototype.reduce Disallowed Disallowed
%TypedArray%.prototype.reduceRight Disallowed Disallowed
%TypedArray%.prototype.reverse Disallowed Disallowed
%TypedArray%.prototype.some Disallowed Disallowed
%TypedArray%.prototype.sort Disallowed Disallowed
%TypedArray%.prototype.copyWithin Disallowed Disallowed
%TypedArray%.prototype.find Disallowed Disallowed
%TypedArray%.prototype.findIndex Disallowed Disallowed
%TypedArray%.prototype.fill Disallowed Disallowed
%TypedArray%.prototype.keys Disallowed Disallowed
%TypedArray%.prototype.values Disallowed Disallowed
%TypedArray%.prototype.entries Disallowed Disallowed
%TypedArray%.prototype[Symbol.iterator] Disallowed Disallowed
%TypedArray%[Symbol.species] Disallowed Disallowed
Table 68. Map
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
constructor arguments Supported Not Supported
constructor requires new Supported Not Supported
constructor accepts null Supported Not Supported
constructor invokes set Supported Not Supported
iterator closing Supported Not Supported
Map.prototype.set returns this Supported Not Supported
-0 key converts to +0 Supported Not Supported
Map.prototype.size Supported Not Supported
Map.prototype.delete Supported Not Supported
Map.prototype.clear Supported Not Supported
Map.prototype.forEach Supported Not Supported
Map.prototype.keys Supported Not Supported
Map.prototype.values Supported Not Supported
Map.prototype.entries Supported Not Supported
Map.prototype[Symbol.iterator] Supported Not Supported
Map.prototype isn't an instance Supported Not Supported
Map iterator prototype chain Supported Not Supported
Map[Symbol.species] Supported Not Supported
Table 69. Set
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
constructor arguments Supported Not Supported
constructor requires new Supported Not Supported
constructor accepts null Supported Not Supported
constructor invokes add Supported Not Supported
iterator closing Supported Not Supported
Set.prototype.add returns this Supported Not Supported
-0 key converts to +0 Supported Not Supported
Set.prototype.size Supported Not Supported
Set.prototype.delete Supported Not Supported
Set.prototype.clear Supported Not Supported
Set.prototype.forEach Supported Not Supported
Set.prototype.keys Supported Not Supported
Set.prototype.values Supported Not Supported
Set.prototype.entries Supported Not Supported
Set.prototype[Symbol.iterator] Supported Not Supported
Set.prototype isn't an instance Supported Not Supported
Set iterator prototype chain Supported Not Supported
Set[Symbol.species] Supported Not Supported
Table 70. WeakMap
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
constructor arguments Disallowed Disallowed
constructor requires new Disallowed Disallowed
constructor accepts null Disallowed Disallowed
constructor invokes set Disallowed Disallowed
frozen objects as keys Disallowed Disallowed
iterator closing Disallowed Disallowed
WeakMap.prototype.set returns this Disallowed Disallowed
WeakMap.prototype.delete Disallowed Disallowed
no WeakMap.prototype.clear method Disallowed Disallowed
.has, .get and .delete methods accept primitives Disallowed Disallowed
WeakMap.prototype isn't an instance Disallowed Disallowed
Table 71. WeakSet
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
constructor arguments Disallowed Disallowed
constructor requires new Disallowed Disallowed
constructor accepts null Disallowed Disallowed
constructor invokes add Disallowed Disallowed
iterator closing Disallowed Disallowed
WeakSet.prototype.add returns this Disallowed Disallowed
WeakSet.prototype.delete Disallowed Disallowed
no WeakSet.prototype.clear method Disallowed Disallowed
.has and .delete methods accept primitives Disallowed Disallowed
WeakSet.prototype isn't an instance Disallowed Disallowed
Table 72. Proxy
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
constructor requires new Disallowed Disallowed
no "prototype" property Disallowed Disallowed
"get" handler Disallowed Disallowed
"get" handler, instances of proxies Disallowed Disallowed
"get" handler invariants Disallowed Disallowed
"set" handler Disallowed Disallowed
"set" handler, instances of proxies Disallowed Disallowed
"set" handler invariants Disallowed Disallowed
"has" handler Disallowed Disallowed
"has" handler, instances of proxies Disallowed Disallowed
"has" handler invariants Disallowed Disallowed
"deleteProperty" handler Disallowed Disallowed
"deleteProperty" handler invariant Disallowed Disallowed
"getOwnPropertyDescriptor" handler Disallowed Disallowed
"getOwnPropertyDescriptor" handler invariants Disallowed Disallowed
"defineProperty" handler Disallowed Disallowed
"defineProperty" handler invariants Disallowed Disallowed
"getPrototypeOf" handler Disallowed Disallowed
"getPrototypeOf" handler invariant Disallowed Disallowed
"setPrototypeOf" handler Disallowed Disallowed
"setPrototypeOf" handler invariant Disallowed Disallowed
"isExtensible" handler Disallowed Disallowed
"isExtensible" handler invariant Disallowed Disallowed
"preventExtensions" handler Disallowed Disallowed
"preventExtensions" handler invariant Disallowed Disallowed
"ownKeys" handler Disallowed Disallowed
"ownKeys" handler invariant Disallowed Disallowed
"apply" handler Disallowed Disallowed
"apply" handler invariant Disallowed Disallowed
"construct" handler Disallowed Disallowed
"construct" handler invariants Disallowed Disallowed
Proxy.revocable Disallowed Disallowed
Array.isArray support Disallowed Disallowed
JSON.stringify support Disallowed Disallowed
Table 73. Reflect
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Reflect.get Disallowed Disallowed
Reflect.set Disallowed Disallowed
Reflect.has Disallowed Disallowed
Reflect.deleteProperty Disallowed Disallowed
Reflect.getOwnPropertyDescriptor Disallowed Disallowed
Reflect.defineProperty Disallowed Disallowed
Reflect.getPrototypeOf Disallowed Disallowed
Reflect.setPrototypeOf Disallowed Disallowed
Reflect.isExtensible Disallowed Disallowed
Reflect.preventExtensions Disallowed Disallowed
Reflect.ownKeys, string keys Disallowed Disallowed
Reflect.ownKeys, symbol keys Disallowed Disallowed
Reflect.apply Disallowed Disallowed
Reflect.construct Disallowed Disallowed
Reflect.construct sets new.target meta-property Disallowed Disallowed
Reflect.construct creates instances from third argument Disallowed Disallowed
Reflect.construct, Array subclassing Disallowed Disallowed
Reflect.construct, RegExp subclassing Disallowed Disallowed
Reflect.construct, Function subclassing Disallowed Disallowed
Reflect.construct, Promise subclassing Disallowed Disallowed
Table 74. Promise
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
constructor requires new Disallowed Disallowed
Promise.prototype isn't an instance Disallowed Disallowed
Promise.all Disallowed Disallowed
Promise.all, generic iterables Disallowed Disallowed
Promise.race Disallowed Disallowed
Promise.race, generic iterables Disallowed Disallowed
Promise[Symbol.species] Disallowed Disallowed
Table 75. Symbol
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Supported Not Supported
typeof support Supported Not Supported
symbol keys are hidden to pre-ES6 code Supported Not Supported
Object.defineProperty support Supported Not Supported
symbols inherit from Symbol.prototype Supported Not Supported
cannot coerce to string or number Not Supported Not Supported
can convert with String() Not Supported Not Supported
new Symbol() throws Supported Not Supported
Object(symbol) Not Supported Not Supported
JSON.stringify ignores symbol primitives Supported Not Supported
JSON.stringify ignores symbol objects Not Supported Not Supported
global symbol registry Supported Not Supported
Table 76. Well-known symbols
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Symbol.hasInstance Disallowed Disallowed
Symbol.isConcatSpreadable Disallowed Disallowed
Symbol.iterator, existence Disallowed Disallowed
Symbol.iterator, arguments object Disallowed Disallowed
Symbol.species, existence Disallowed Disallowed
Symbol.species, Array.prototype.concat Disallowed Disallowed
Symbol.species, Array.prototype.filter Disallowed Disallowed
Symbol.species, Array.prototype.map Disallowed Disallowed
Symbol.species, Array.prototype.slice Disallowed Disallowed
Symbol.species, Array.prototype.splice Disallowed Disallowed
Symbol.species, RegExp.prototype[Symbol.split] Disallowed Disallowed
Symbol.species, Promise.prototype.then Disallowed Disallowed
Symbol.replace Disallowed Disallowed
Symbol.search Disallowed Disallowed
Symbol.split Disallowed Disallowed
Symbol.match Disallowed Disallowed
Symbol.match, RegExp constructor Disallowed Disallowed
Symbol.match, String.prototype.startsWith Disallowed Disallowed
Symbol.match, String.prototype.endsWith Disallowed Disallowed
Symbol.match, String.prototype.includes Disallowed Disallowed
Symbol.toPrimitive Disallowed Disallowed
Symbol.toStringTag Disallowed Disallowed
Symbol.toStringTag affects existing built-ins Disallowed Disallowed
Symbol.toStringTag, new built-ins Disallowed Disallowed
Symbol.toStringTag, misc. built-ins Disallowed Disallowed
Symbol.unscopables Disallowed Disallowed
Table 77. Object static methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.assign Supported Not Supported
Object.is Supported Not Supported
Object.getOwnPropertySymbols Supported Not Supported
Object.setPrototypeOf Not Supported Not Supported
Table 78. Function "name" property
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
function statements Supported Supported
function expressions Supported Supported
new Function Not Supported Not Supported
bound functions Not Supported Not Supported
variables (function) Supported Not Supported
object methods (function) Supported Not Supported
accessor properties Not Supported Not Supported
shorthand methods Supported Not Supported
shorthand methods (no lexical binding) Supported Not Supported
symbol-keyed methods Not Supported Not Supported
class statements Supported Not Supported
class expressions Supported Not Supported
variables (class) Supported Not Supported
object methods (class) Not Supported Not Supported
class prototype methods Supported Not Supported
class static methods Supported Not Supported
isn't writable, is configurable Not Supported Not Supported
Table 79. String static methods
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
String.raw Supported Not Supported
String.fromCodePoint Supported Not Supported
Table 80. String.prototype methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
String.prototype.codePointAt Supported Supported
String.prototype.normalize Supported Supported
String.prototype.repeat Supported Supported
String.prototype.startsWith Supported Supported
String.prototype.startsWith throws on RegExp Not Supported Not Supported
String.prototype.endsWith Supported Supported
String.prototype.endsWith throws on RegExp Not Supported Not Supported
String.prototype.includes Supported Supported
String.prototype[Symbol.iterator] Supported Not Supported
String iterator prototype chain Supported Not Supported
Table 81. RegExp.prototype properties
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
RegExp.prototype.flags Supported Not Supported
RegExp.prototype[Symbol.match] Not Supported Not Supported
RegExp.prototype[Symbol.replace] Supported Not Supported
RegExp.prototype[Symbol.split] Supported Not Supported
RegExp.prototype[Symbol.search] Not Supported Not Supported
RegExp[Symbol.species] Supported Not Supported
Table 82. Array static methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Array.from, array-like objects Supported Not Supported
Array.from, generator instances Disallowed Disallowed
Array.from, generic iterables Supported Not Supported
Array.from, instances of generic iterables Supported Not Supported
Array.from map function, array-like objects Supported Not Supported
Array.from map function, generator instances Disallowed Disallowed
Array.from map function, generic iterables Supported Not Supported
Array.from map function, instances of iterables Supported Not Supported
Array.from, iterator closing Supported Not Supported
Array.of Supported Not Supported
Array[Symbol.species] Supported Not Supported
Table 83. Array.prototype methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Array.prototype.copyWithin Supported Not Supported
Array.prototype.find Supported Not Supported
Array.prototype.findIndex Supported Not Supported
Array.prototype.fill Supported Not Supported
Array.prototype.keys Supported Not Supported
Array.prototype.values Supported Not Supported
Array.prototype.entries Supported Not Supported
Array.prototype[Symbol.iterator] Supported Not Supported
Array iterator prototype chain Supported Not Supported
Array.prototype[Symbol.unscopables] Supported Not Supported
Table 84. Number properties
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Number.isFinite Supported Not Supported
Number.isInteger Supported Not Supported
Number.isSafeInteger Supported Not Supported
Number.isNaN Supported Not Supported
Number.parseFloat Disallowed Disallowed
Number.parseInt Disallowed Disallowed
Number.EPSILON Supported Not Supported
Number.MIN_SAFE_INTEGER Supported Not Supported
Number.MAX_SAFE_INTEGER Supported Not Supported
Table 85. Math methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Math.clz32 Supported Not Supported
Math.imul Supported Not Supported
Math.sign Supported Not Supported
Math.log10 Supported Not Supported
Math.log2 Supported Not Supported
Math.log1p Supported Not Supported
Math.expm1 Supported Not Supported
Math.cosh Supported Not Supported
Math.sinh Supported Not Supported
Math.tanh Supported Not Supported
Math.acosh Supported Not Supported
Math.asinh Supported Not Supported
Math.atanh Supported Not Supported
Math.trunc Supported Not Supported
Math.fround Supported Not Supported
Math.cbrt Supported Not Supported
Math.hypot Supported Not Supported
Table 86. Date.prototype[Symbol.toPrimitive]
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Date.prototype[Symbol.toPrimitive] Supported Not Supported
Table 87. Array is subclassable
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
length property (accessing) Disallowed Disallowed
length property (setting) Disallowed Disallowed
correct prototype chain Disallowed Disallowed
Array.isArray support Disallowed Disallowed
Array.prototype.concat Disallowed Disallowed
Array.prototype.filter Disallowed Disallowed
Array.prototype.map Disallowed Disallowed
Array.prototype.slice Disallowed Disallowed
Array.prototype.splice Disallowed Disallowed
Array.from Disallowed Disallowed
Array.of Disallowed Disallowed
Table 88. RegExp is subclassable
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
correct prototype chain Disallowed Disallowed
RegExp.prototype.exec Disallowed Disallowed
RegExp.prototype.test Disallowed Disallowed
Table 89. Function is subclassable
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
can be called Disallowed Disallowed
correct prototype chain Disallowed Disallowed
can be used with "new" Disallowed Disallowed
Function.prototype.call Disallowed Disallowed
Function.prototype.apply Disallowed Disallowed
Function.prototype.bind Disallowed Disallowed
Table 90. Promise is subclassable
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functionality Disallowed Disallowed
correct prototype chain Disallowed Disallowed
Promise.all Disallowed Disallowed
Promise.race Disallowed Disallowed
Table 91. Miscellaneous subclassables
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Boolean is subclassable Disallowed Disallowed
Number is subclassable Disallowed Disallowed
String is subclassable Disallowed Disallowed
Error is subclassable Disallowed Disallowed
Map is subclassable Disallowed Disallowed
Set is subclassable Disallowed Disallowed
Table 92. Prototype of bound functions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic functions Disallowed Disallowed
generator functions Disallowed Disallowed
arrow functions Disallowed Disallowed
classes Disallowed Disallowed
subclasses Disallowed Disallowed
Table 93. Proxy, internal 'get' calls
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
ToPrimitive Disallowed Disallowed
CreateListFromArrayLike Disallowed Disallowed
instanceof operator Supported Disallowed
HasBinding Disallowed Disallowed
CreateDynamicFunction Disallowed Disallowed
ClassDefinitionEvaluation Disallowed Disallowed
IteratorComplete, IteratorValue Disallowed Disallowed
ToPropertyDescriptor Disallowed Disallowed
Object.assign Disallowed Disallowed
Object.defineProperties Disallowed Disallowed
Function.prototype.bind Disallowed Disallowed
Error.prototype.toString Disallowed Disallowed
String.raw Disallowed Disallowed
RegExp constructor Disallowed Disallowed
RegExp.prototype.flags Disallowed Disallowed
RegExp.prototype.test Disallowed Disallowed
RegExp.prototype.toString Disallowed Disallowed
RegExp.prototype[Symbol.match] Disallowed Disallowed
RegExp.prototype[Symbol.replace] Disallowed Disallowed
RegExp.prototype[Symbol.search] Disallowed Disallowed
RegExp.prototype[Symbol.split] Disallowed Disallowed
Array.from Disallowed Disallowed
Array.prototype.concat Disallowed Disallowed
Array.prototype iteration methods Disallowed Disallowed
Array.prototype.pop Disallowed Disallowed
Array.prototype.reverse Disallowed Disallowed
Array.prototype.shift Disallowed Disallowed
Array.prototype.splice Disallowed Disallowed
Array.prototype.toString Disallowed Disallowed
JSON.stringify Disallowed Disallowed
Promise resolve functions Disallowed Disallowed
String.prototype.match Disallowed Disallowed
String.prototype.replace Disallowed Disallowed
String.prototype.search Disallowed Disallowed
String.prototype.split Disallowed Disallowed
Date.prototype.toJSON Disallowed Disallowed
Table 94. Proxy, internal 'set' calls
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.assign Disallowed Disallowed
Array.from Disallowed Disallowed
Array.of Disallowed Disallowed
Array.prototype.copyWithin Disallowed Disallowed
Array.prototype.fill Disallowed Disallowed
Array.prototype.pop Disallowed Disallowed
Array.prototype.push Disallowed Disallowed
Array.prototype.reverse Disallowed Disallowed
Array.prototype.shift Disallowed Disallowed
Array.prototype.splice Disallowed Disallowed
Array.prototype.unshift Disallowed Disallowed
Table 95. Proxy, internal 'defineProperty' calls
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
[[Set]] Disallowed Disallowed
SetIntegrityLevel Disallowed Disallowed
Table 96. Proxy, internal 'deleteProperty' calls
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Array.prototype.copyWithin Disallowed Disallowed
Array.prototype.pop Disallowed Disallowed
Array.prototype.reverse Disallowed Disallowed
Array.prototype.shift Disallowed Disallowed
Array.prototype.splice Disallowed Disallowed
Array.prototype.unshift Disallowed Disallowed
Table 97. Proxy, internal 'getOwnPropertyDescriptor' calls
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
[[Set]] Disallowed Disallowed
Object.assign Disallowed Disallowed
Object.prototype.hasOwnProperty Disallowed Disallowed
Function.prototype.bind Disallowed Disallowed
Table 98. Proxy, internal 'ownKeys' calls
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
SetIntegrityLevel Disallowed Disallowed
TestIntegrityLevel Disallowed Disallowed
SerializeJSONObject Disallowed Disallowed
Table 99. Object static methods accept primitives
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.getPrototypeOf Disallowed Disallowed
Object.getOwnPropertyDescriptor Disallowed Disallowed
Object.getOwnPropertyNames Disallowed Disallowed
Object.seal Disallowed Disallowed
Object.freeze Disallowed Disallowed
Object.preventExtensions Disallowed Disallowed
Object.isSealed Disallowed Disallowed
Object.isFrozen Disallowed Disallowed
Object.isExtensible Disallowed Disallowed
Object.keys Disallowed Disallowed
Table 100. Own property order
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.keys Disallowed Disallowed
Object.getOwnPropertyNames Disallowed Disallowed
Object.assign Disallowed Disallowed
JSON.stringify Disallowed Disallowed
JSON.parse Disallowed Disallowed
Reflect.ownKeys, string key order Disallowed Disallowed
Reflect.ownKeys, symbol key order Disallowed Disallowed
Table 101. Updated identifier syntax
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
var ⸯ; Disallowed Disallowed
var ð ‹€; Disallowed Disallowed
no escaped reserved words as identifiers Disallowed Disallowed
Table 102. Non-strict function semantics
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
hoisted block-level function declaration Disallowed Disallowed
labeled function statements Disallowed Disallowed
function statements in if-statement clauses Disallowed Disallowed
Table 103. __proto__ in object literals
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
basic support Disallowed Disallowed
multiple __proto__ is an error Disallowed Disallowed
not a computed property Disallowed Disallowed
not a shorthand property Disallowed Disallowed
not a shorthand method Disallowed Disallowed
Table 104. Object.prototype.__proto__
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
get prototype Disallowed Disallowed
set prototype Disallowed Disallowed
absent from Object.create(null) Disallowed Disallowed
present in hasOwnProperty() Disallowed Disallowed
correct property descriptor Disallowed Disallowed
present in Object.getOwnPropertyNames() Disallowed Disallowed
Table 105. String.prototype HTML methods
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
existence Disallowed Disallowed
tags' names are lowercase Disallowed Disallowed
quotes in arguments are escaped Disallowed Disallowed
Table 106. RegExp.prototype.compile
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
basic functionality Disallowed Disallowed
returns this Disallowed Disallowed
Table 107. RegExp syntax extensions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
hyphens in character sets Disallowed Disallowed
invalid character escapes Disallowed Disallowed
invalid control-character escapes Disallowed Disallowed
invalid Unicode escapes Disallowed Disallowed
invalid hexadecimal escapes Disallowed Disallowed
incomplete patterns and quantifiers Disallowed Disallowed
octal escape sequences Disallowed Disallowed
invalid backreferences become octal escapes Disallowed Disallowed

ECMAScript 2009 (ES5) features

Table 108. Object/array literal extensions
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Getter accessors Supported Supported
Setter accessors Supported Supported
Trailing commas in object literals Supported Supported
Trailing commas in array literals Supported Supported
Reserved words as property names Supported Supported
Table 109. Object static methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Object.create Supported Supported
Object.defineProperty Supported Supported
Object.defineProperties Supported Supported
Object.getPrototypeOf Supported Supported
Object.keys Supported Supported
Object.seal Supported Supported
Object.freeze Supported Supported
Object.preventExtensions Supported Supported
Object.isSealed Supported Supported
Object.isFrozen Supported Supported
Object.isExtensible Supported Supported
Object.getOwnPropertyDescriptor Supported Supported
Object.getOwnPropertyNames Supported Supported
Table 110. Array methods
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
Array.isArray Supported Supported
Array.prototype.indexOf Supported Supported
Array.prototype.lastIndexOf Supported Supported
Array.prototype.every Supported Supported
Array.prototype.some Supported Supported
Array.prototype.forEach Supported Supported
Array.prototype.map Supported Supported
Array.prototype.filter Supported Supported
Array.prototype.reduce Supported Supported
Array.prototype.reduceRight Supported Supported
Array.prototype.sort: compareFn must be function or undefined Not Supported Not Supported
Array.prototype.sort: compareFn may be explicit undefined Supported Supported
Table 111. String properties and methods
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Property access on strings Supported Supported
String.prototype.split Supported Not Supported
String.prototype.trim Supported Supported
Table 112. Date methods
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Date.prototype.toISOString Supported Supported
Date.now Supported Supported
Date.prototype.toJSON Not Supported Not Supported
Table 113. Immutable globals
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
undefined Supported Supported
NaN Supported Supported
Infinity Supported Supported
Table 114. Number methods
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Number.prototype.toExponential rounds properly Supported Supported
Number.prototype.toExponential throws on ±Infinity fractionDigits Supported Supported
Number.prototype.toExponential does not throw on edge cases Supported Supported
Table 115. Strict mode
FeatureECMAScript 2021 (ES12) modeES5 Standards mode
reserved words Disallowed Disallowed
"this" is undefined in functions Disallowed Disallowed
"this" is not coerced to object in primitive methods Disallowed Disallowed
"this" is not coerced to object in primitive accessors Disallowed Disallowed
legacy octal is a SyntaxError Disallowed Disallowed
assignment to unresolvable identifiers is a ReferenceError Disallowed Disallowed
assignment to eval or arguments is a SyntaxError Disallowed Disallowed
assignment to non-writable properties is a TypeError Disallowed Disallowed
eval or arguments bindings is a SyntaxError Disallowed Disallowed
arguments.caller removed or is a TypeError Disallowed Disallowed
arguments.callee is a TypeError Disallowed Disallowed
(function(){}).caller and (function(){}).arguments is a TypeError Disallowed Disallowed
arguments is unmapped Disallowed Disallowed
eval() can't create bindings Disallowed Disallowed
deleting bindings is a SyntaxError Disallowed Disallowed
deleting non-configurable properties is a TypeError Disallowed Disallowed
"with" is a SyntaxError Disallowed Disallowed
repeated parameter names is a SyntaxError Disallowed Disallowed
function expressions with matching name and argument are valid Disallowed Disallowed
Table 116. Function.prototype.bind
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
Function.prototype.bind Supported Supported
Table 117. JSON
Feature ECMAScript 2021 (ES12) mode ES5 Standards mode
JSON Supported Supported