Turn ES6 code into readable vanilla ES5 with source maps
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6to5 turns ES6 code into vanilla ES5, so you can use ES6 features today.

  • Readable - formatting is retained if possible so your generated code is as similar as possible.
  • Extensible - with a large range of plugins and browser support.
  • Lossless - source map support so you can debug your compiled code with ease.
  • Compact - maps directly to the equivalent ES5 with no runtime*.


It's as easy as:

$ npm install -g 6to5

Table of Contents





Compile the file script.js and output it to stdout.

$ 6to5 script.js

Compile the file script.js and output it to script-compiled.js.

$ 6to5 script.js --out-file script-compiled.js

Compile the file script.js and output it to script-compiled.js and save a source map to script-compiled.js.map.

$ 6to5 script.js --source-maps --out-file script-compiled.js

Compile the file script.js and output it to script-compiled.js with a source map embedded in a comment at the bottom.

$ 6to5 script.js --source-maps-inline --out-file script-compiled.js

Compile the entire src directory and output it to the lib directory.

$ 6to5 src --out-dir lib

Compile the entire src directory and output it to the one concatenated file.

$ 6to5 src --out-file script-compiled.js

Pipe a file in via stdin and output it to script-compiled.js

$ 6to5 --out-file script-compiled.js < script.js


Launch a repl.

$ 6to5-node

Evaluate code.

$ 6to5-node -e "class Test { }"

Compile and run test.js.

$ 6to5-node test


var to5 = require("6to5");

var result = to5.transform("code();", options);

to5.transformFileSync("filename.js", options).code;

to5.transformFile("filename.js", options, function (err, result) {

  // Filename for use in errors etc.
  // Default: "unknown"
  filename: "filename",

  // List of transformers to EXCLUDE.
  // Run `6to5 --help` to see a full list of transformers.
  blacklist: [],

  // List of transformers to ONLY use.
  // Run `6to5 --help` to see a full list of transformers.
  whitelist: [],

  // Module formatter to use
  // Run `6to5 --help` to see a full list of module formatters.
  // Default: "common"
  modules: "common",

  // If truthy, adds a `map` property to returned output.
  // If set to "inline", a comment with a sourceMappingURL directive is added to
  // the bottom of the returned code.
  // Default: false
  sourceMap: true,

  // Set `file` on returned source map.
  // Default: `filename` option.
  sourceMapName: "filename",

  // Set `sources[0]` on returned source map.
  // Default: `filename` option.
  sourceFileName: "filename"

Require hook

All subsequent files required by node with the extensions .es6 and .js will be transformed by 6to5. The polyfill specified in Polyfill is also required.


NOTE: By default all requires to node_modules will be ignored. You can override this by passing an ignore regex via:


You can also customise the file extensions that the require hook will use via:

  // optional ignore regex
  ignoreRegex: /regex/,

  // this will remove the currently hooked extensions of .es6 and .js so you'll
  // have to add them back if you want them to be used again
  extensions: [".js", ".es6"]


You can build a browser version of the compiler by running the following in the 6to5 directory:

$ make build

This will output the files dist/6to5.js and dist/6to5.min.js.

Just include one of those in the browser and access the transform method via the global to5.

to5("class Test {}").code;


To test 6to5 in your browser run:

$ make test-browser

And open test/browser.html in your browser if it doesn't open automatically.


See Modules - Common for documentation on the default module formatting.

Alternatively see Modules for all other supported module formatting types.



6to5 does not include a runtime nor polyfill and it's up to the developer to include one in compiled browser code.


A polyfill is included with the 6to5 module that can be included in node like so:


This is simply a wrapper around the regenerator runtime and the es6-shim and es6-symbol polyfills.


If you're planning on using 6to5 output in the browser then it's up to you to include polyfills. es6-symbol and es6-shim fill the vast majority of polyfill concerns.


A polyfill is required for for-of functionality that implements Symbol and adds prototype[Symbol.iterator] behaviour to built-ins. Using the polyfills specified in polyfill suffices.


Built-in classes such as Date, Array and DOM cannot be subclassed due to limitations in ES5 implementations.

If you're inheriting from a class then static properties are inherited from it via __proto__, this is widely supported but you may run into problems with much older browsers.


The regenerator runtime and an ES6 polyfill are required in order for generators to work.



The fundamental concept behind 6to5 is that the generated code must be close as possible to the original, retaining all the same formatting and readability.

Many other transpilers are just concerned with making the code work while 6to5 is concerned with making sure it works and is readable at the same time.

For example, given the following array comprehension:

var seattlers = [for (c of customers) if (c.city == "Seattle") { name: c.name, age: c.age }];

is generated to the following with 6to5:

var seattlers = customers.filter(function (c) {
  return c.city == "Seattle";
}).map(function (c) {
  return {
    name: c.name,
    age: c.age

The following is what Traceur generates:

var seattlers = (function() {
  var c;
  var $__20 = 0,
      $__21 = [];
  for (var $__22 = customers[$traceurRuntime.toProperty(Symbol.iterator)](),
      $__23; !($__23 = $__22.next()).done; ) {
    c = $__23.value;
    if (c.city == "Seattle")
      $traceurRuntime.setProperty($__21, $__20++, {
        name: c.name,
        age: c.age
  return $__21;

As you can tell, it's not very pretty, unreadable even. Instead of mapping directly to a runtime, like other transpilers, 6to5 maps directly to the equivalent ES5.

I'm not saying 6to5 is for everyone or even suited for everything. Traceur is better suited if you'd like a full ES6 environment with polyfills and all.

Comparison to other transpilers

6to5 Traceur esnext es6now es6-transpiler jstransform
No runtime
Source maps
No compiler global pollution
Arrow functions
Computed property names
Default parameters
Let scoping
Property method assignment
Property name shorthand
Rest parameters
Template literals
Unicode regex


Traceur requires quite a bulky runtime (~75KB) and produces quite verbose code. While this can be trimmed down by selectively building the runtime, it's an unneccesary step when a runtime can be eliminated entirely.


es6now doesn't output sourcemaps. This is cited as a positive as line-to-line mapping is the goal. This however obviously doesn't retain column mapping resulting in the output code not being very pleasant.


The es6-transpiler compiler requires shims to operate which pollutes the global scope resulting in possible collisions.

es6-transpiler maps line-by-line, just like es6now, this results in the same issues such as lack of column information and unpleasant code output.