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Language Features in Hermes

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


  • Symbols (including most well-known Symbols)
  • Iteration (with [Symbol.iterator])
  • for..of loops
  • Destructuring assignment (with array and object "rest" properties)
  • Template string literals
  • Generators (function* and yield)
  • TypedArrays
  • Arrow functions
  • All ES6 JS library functions
    • Set/Map
    • WeakSet/WeakMap
    • ES6 String searching functions
    • ES6 Array searching functions

In Progress

  • let and const (block scoped variables, with support for the temporal dead zone)
  • Classes and method definitions
  • Computed property keys on object literals
  • ES modules (import and export)

Excluded From Support

  • Reflection (Reflect and Proxy)
  • Realms
  • with statements
  • Local mode eval() (use and introduce local variables)
  • /u (Unicode) and /y (sticky) flags in RegExp
  • Intl API
  • Symbol.species and its interactions with JS library functions
  • Symbol.unscopables (Hermes does not support with)
  • Other features added to ECMAScript after ES6 not listed under "Supported"

Miscellaneous Incompatibilities

  • Function.prototype.toString cannot show source because Hermes executes from bytecode
  • arguments changes in non-strict mode will not sync with named parameters
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