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Features.md

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id language-features
title Language Features

Hermes plans to target ECMAScript 2015 (ES6), with some carefully considered exceptions.

Supported

  • All ES6 JS library functions
    • ES6 Array searching functions
    • ES6 String searching functions
    • Set/Map
    • WeakSet/WeakMap
  • Array spread
  • Arrow functions
  • BigInt
  • Destructuring assignment (with array and object "rest" properties)
  • ES6 Promise (with incompatibilities documented below)
  • for..of loops
  • Generators (function* and yield)
  • Intl, or Internationalization APIs
  • Iteration (with [Symbol.iterator])
  • Object rest/spread
  • Optional chaining and nullish coalescing (?. and ??)
  • Reflection (Reflect and Proxy) starting from v0.7.0
  • Shorthand property and computed property on object literals
  • Symbols (including most well-known Symbols)
  • Template string literals
  • TypedArrays

In Progress

  • Async function (async and await).
  • ES modules (import and export)
  • Intl API glue has been added to enable community contribution of a complete, spec-compliant implementation on multiple platforms
  • Symbol.prototype.description (it's not fully spec-conformant yet. Symbol().description should be undefined but it's currently '').
  • WeakRef

Planned

  • Block scoped variables (let and const), with support for the temporal dead zone
  • Classes and method definitions
  • Expanded Intl functionality (e.g., DisplayNames, ListFormat, PluralRules, RelativeTimeFormat, and Locale)
  • RegExp match indices and named capture groups

Excluded From Support

  • Local mode eval() (use and introduce local variables)
  • Other features added to ECMAScript after ES6 not listed under "Supported"
  • Realms
  • Symbol.species and its interactions with JS library functions
  • Symbol.unscopables (Hermes does not support with)
  • use of constructor property when creating new Arrays in Array.prototype methods
  • with statements

Miscellaneous Incompatibilities

  • arguments changes in non-strict mode will not sync with named parameters
  • Function.prototype.toString cannot show source because Hermes executes from bytecode
  • Promise is implemented by pre-compiling the JS polyfill from RN as the internal bytecode to preserve the current interoperation, hence its conformance to the spec is up to conformance of the polyfill.
    • In case you want to bring in your own Promise and opt-out Hermes', you can turn it off by passing -Xes6-promise=0 in CLI or setting withES6Promise(false) in the runtime configs.