README.md

babel-plugin-transform-es2015-for-of

Compile ES2015 for...of to ES5

Installation

npm install --save-dev babel-plugin-transform-es2015-for-of

Usage

Via .babelrc (Recommended)

.babelrc

// without options
{
  "plugins": ["transform-es2015-for-of"]
}

// with options
{
  "plugins": [
    ["transform-es2015-for-of", {
      "loose": true
    }]
  ]
}

Via CLI

babel --plugins transform-es2015-for-of script.js

Via Node API

require("babel-core").transform("code", {
  plugins: ["transform-es2015-for-of"]
});

Options loose

Abrupt completions

In loose mode an iterators return method will not be called on abrupt completions caused by thrown errors.

Please see google/traceur-compiler#1773 and babel/babel#838 for more information.

Arrays

Under loose mode the forOf transformer will output more verbose iteration code.

For example the following:

for (var i of foo) {}

is normally output as:

for (var _iterator = foo[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
  var i = _step.value;
}

Under loose mode however it's output as:

for (var _iterator = foo, _isArray = Array.isArray(_iterator), _i = 0, _iterator = _isArray ? _iterator : _iterator[Symbol.iterator]();;) {
  var i;
  if (_isArray) {
    if (_i >= _iterator.length) break;
    i = _iterator[_i++];
  } else {
    _i = _iterator.next();
    if (_i.done) break;
    i = _i.value;
  }
}

The result is that arrays are put in a fast path, heavily increasing performance. All other iterables will continue to work fine but array iteration will be significantly faster.

Optimization

If a basic array is used, Babel will compile the for-of loop down to a regular for loop.

In

for (let a of [1,2,3]) {}

Out

var _arr = [1, 2, 3];
for (var _i = 0; _i < _arr.length; _i++) {
  var a = _arr[_i];
}