@babel/plugin-transform-for-of

@babel/plugin-transform-for-of

Compile ES2015 for...of to ES5

Example

In

for (var i of foo) {}

Out

var _iteratorNormalCompletion = true;
var _didIteratorError = false;
var _iteratorError = undefined;

try {
  for (var _iterator = foo[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
    var i = _step.value;
  }
} catch (err) {
  _didIteratorError = true;
  _iteratorError = err;
} finally {
  try {
    if (!_iteratorNormalCompletion && _iterator.return != null) {
      _iterator.return();
    }
  } finally {
    if (_didIteratorError) {
      throw _iteratorError;
    }
  }
}

Installation

npm install --save-dev @babel/plugin-transform-for-of

Usage

Via .babelrc (Recommended)

.babelrc

Without options:

{
  "plugins": ["@babel/plugin-transform-for-of"]
}

With options:

{
  "plugins": [
    ["@babel/plugin-transform-for-of", {
      "loose": true, // defaults to false
      "assumeArray": true // defaults to false
    }]
  ]
}

Via CLI

babel --plugins @babel/plugin-transform-for-of script.js

Via Node API

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

Options

loose

boolean, defaults to false

In loose mode, arrays are put in a fast path, thus heavily increasing performance. All other iterables will continue to work fine.

Example

In

for (var i of foo) {}

Out

for (var _iterator = foo, _isArray = Array.isArray(_iterator), _i = 0, _iterator = _isArray ? _iterator : _iterator[Symbol.iterator]();;) {
  var _ref;

  if (_isArray) {
    if (_i >= _iterator.length) break;
    _ref = _iterator[_i++];
  } else {
    _i = _iterator.next();
    if (_i.done) break;
    _ref = _i.value;
  }

  var i = _ref;
}

Abrupt completions

In loose mode an iterator's 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.

assumeArray

boolean, defaults to false

This will apply the optimization shown below to all for-of loops by assuming that all loops are arrays.

Can be useful when you just want a for-of loop to represent a basic for loop over an array.

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];
}