es6 -> es5


es6 -> es5


  • different output than traceur-compiler:
    1. no runtime library (only polyfills if needed)
    2. no try/catch for block binding
    3. spread:
    * via `.concat`
    * should respect Iterator protocol
    * spread is operator, not a function
    1. minimal count of temporary variables
    2. termination stage for temporary variables
    3. es6 RegExp support
    4. and others
  • output should support Closure Compiler
  • line-to-line input/output mapping
  • classes
  • generator comprehensions
  • destructuring (with default values)
  • block binding (let / const)
    • loops: fresh lexical environment per iteration
  • function default parameters and rest
  • arrow functions
  • spread (with iterator protocol)
  • for-of (with iterator protocol)
  • array comprehensions (with iterator protocol)
  • string templates (with tags support)
  • object literals:
    • methods
    • shorthands
    • computed properties
  • binary/octal numericLiteral
  • unicode code point escapes
  • RegExp:
    • 'y' flag support (in runtime via polyfill)
    • 'u' flag support:
      • full transpiler-time support: [negative] astral symbols (surrogate pairs) ranges, \D, \W, '.' etc captures astral symbols
      • partial runtime support see: Not supported

Static scope analysis and transpilation of ES6 block scoped const and let variables to ES3 based on olov/defs.

  • modules
  • generators
  • symbols
  • RegExp:
    • 'u' flag in runtime (via polyfill) for the newly generated patterns:
      1. \D, \W, '.' etc not supported
      2. negative astral symbols (surrogate pairs) ranges
var obj = {a: 1, b: 2, c: 3};
var Symbol_iterator = typeof Symbol !== 'undefined' && Symbol.iterator || "@@iterator";
obj[Symbol_iterator] = function() {
    var iterableObject = this;
    var keys = ["a", "b", "c"];
    return {
        nextfunction() {
            var currentKey = keys.shift();
            return {
                value: currentKey ? iterableObject[currentKey] : void 0
                , done: !currentKey

Install using npm

npm install es6-transpiler

Or just using Grunt task (see below).

For the output code works you need an implementation of Object.create in the target browser. You can get it here: es5-shim or copy and past this code:

if(!Object.create)Object.create = function(_prototype) {//[Warning!!!]This is PURE and UNSAFE implementation of Object.create
    var Type = function () {};
    Type.prototype = _prototype;
    var _object = new Type();
    _object.__proto__ = _prototype;
    return _object;

Grunt task can be fount here:

Install: npm install grunt-es6-transpiler


  "es6-transpiler": {
    test: {
        src: 'test.js'
        , dest: 'test.es5.js'

Gulp task can be fount here:

Install: npm install --save-dev gulp-es6-transpiler


var gulp = require('gulp');
var es6transpiler = require('gulp-es6-transpiler');
gulp.task('default', function () {

Run it as es6toes5 <input file>. Or node --harmony es6toes5 <input file>. Also you can run a compiled es5 version node build/es5/es6toes5 <input file>. The errors (if any) will go to stderr, the transpiled source to stdout, so redirect it like es6toes5 file.js > output.js.


Options is:

    filename: string // input file
    src: string // input source if not filename
    outputToConsole: boolean // if true -> result would be outputted to console
    outputFilename: string // if specific -> result would be written to file

Other options below in "Options" section.

var es6tr = require("./es6-transpiler");
var result ={filename: "test.js"});

result object is:

    src: string or "" // on success
    errors: array of error messages or [] // on errors
    stats: statistics object
    ast: transformed ast // ast tree from esprima
    getNeedfulList: <function: Array.<string>> // list of necessary polyfills
    getNeedfulLib: <function: string> // text of necessary polyfills
    getFullLib: <function: string> // text of all available polyfills

Example of options object:

    //described above:
    //"filename" or "src": "string"
    //outputToConsole: false
    //outputFilename: true

    "environments": ["node", "browser"],

    "globals": {
        "my": false,
        "hat": true
    "disallowVars": false,
    "disallowDuplicated": true,
    "disallowUnknownReferences": true,
    "includePolyfills": <boolean> | <"full">,
    "polyfillsSeparator": <string>

globals lets you list your program's globals, and indicate whether they are writable (true) or read-only (false), just like jshint.

environments lets you import a set of pre-defined globals, here node and browser. These default environments are borrowed from jshint (see jshint_globals/vars.js).

disallowVars (defaults to false) can be enabled to make usage of var an error.

disallowDuplicated (defaults to true) errors on duplicated var definitions in the same function scope.

disallowUnknownReferences (defaults to true) errors on references to unknown global variables.

includePolyfills (defaults to false) insert polyfills in the output file. true - insert only the necessary polyfills. "full" - insert all available polyfills.

polyfillsSeparator (default - empty string) any string that should be inserted before polyfills library.

MIT, see LICENSE file.

See tests

es6-transpiler.js strives to transpile your program as true to the ES6 semantics as possible, while being as maximally non-intrusive as possible.

es6-transpiler.js detects the vast majority of cases where a variable is referenced prior to its declaration. The one case it cannot detect is the following:

function printx() { console.log(x); }
printx(); // illegal 
let x = 1;
printx(); // legal 

The first call to printx is not legal because x hasn't been initialized at that point of time, which is impossible to catch reliably with statical analysis. v8 --harmony will detect and error on this via run-time checking. es6-transpiler.js will happily transpile this example (let => var and that's it), and the transpiled code will print undefined on the first call to printx. This difference should be a very minor problem in practice.

  1. Generators support
  2. Modules support
  3. 'pre-es6-node10', 'pre-es6-chrome20' and 'pre-es6-ff24' output modes