@babel/plugin-transform-for-of

NOTE: This plugin is included in @babel/preset-env

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

With a configuration file (Recommended)

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").transformSync("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.

⚠️ Consider migrating to the top level skipForOfIteratorClosing assumption.

// babel.config.json
{
  "assumptions": {
    "skipForOfIteratorClosing": true
  }
}

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

Under the skipForOfIteratorClosing assumption, 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.

allowArrayLike

boolean, defaults to false

Added in: v7.10.0

This option allows for-of to be used with array-like objects.

An array-like object is an object with a length property: for example, { 0: "a", 1: "b", length: 2 }. Note that, like real arrays, array-like objects can have “holes”: { 1: "a", length: 3 } is equivalent to [ (hole), "a", (hole) ].

While it is not spec-compliant to iterate array-like objects as if they were arrays, there are many objects that would be iterables in modern browsers with Symbol.iterator support. Some notable examples are the DOM collections, like document.querySelectorAll("img.big"), which are the main use case for this option.

Please note that Babel allows iterating arguments in old engines even if this option is disabled, because it’s defined as iterable in the ECMAScript specification.

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];
}

You can read more about configuring plugin options here