Language Features in Hermes

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

Supported

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)
Intl API glue for Android has been added, in order to enable community contribution of a complete, spec-compliant implementation.
Reflection (Reflect and Proxy) is opt-in until it is stablilized.

Excluded From Support

Realms
with statements
Local mode eval() (use and introduce local variables)
Symbol.species and its interactions with JS library functions
use of constructor property when creating new Arrays in Array.prototype methods
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