eslint-plugin-no-for-each

What it does? Fix forEach, for in, and for of loops.

Before:

After:

['error', 'cache-length']

['error']

https://jsperf.com/foreach-vs-for-loop-vs-for-in-vs-for-of-vs-babel-for-of

Installation

You'll first need to install ESLint:

$ npm i eslint --save-dev

Next, install eslint-plugin-no-for-each:

$ npm install eslint-plugin-no-for-each --save-dev

Note: If you installed ESLint globally (using the -g flag) then you must also install eslint-plugin-no-for-each globally.

Configuration

Add no-for-each to the plugins section of your .eslintrc configuration file. Then configure the rules you want to use under the rules section.

• default: 1 or ["error"]
• cache-length: 2

{
  "plugins": [
    "no-for-each"
  ],
  "rules": {
    "no-for-each/no-for-each": 2,
    "no-for-each/no-for-of": 2,
    "no-for-each/no-for-in": 2,
  }
}

two additional WIP rules

"rules": {
  "...": "...",
  "no-for-each/cache-for-length": 2,
  "no-excessive-blank-lines": 2
}

Resources

• https://www.kenneth-truyers.net/2016/05/27/writing-custom-eslint-rules/
• https://github.com/airbnb/javascript/issues/851
• https://insideops.wordpress.com/2015/12/08/creating-custom-rules-for-eslint/
• https://medium.com/tumblbug-engineering/creating-an-eslint-plugin-87f1cb42767f#.z5t30ge87
• http://stackoverflow.com/questions/34130718/how-to-create-customized-eslint-rules/34186003
• http://stackoverflow.com/questions/38278273/eslint-code-vs-whitespace-values-for-fixable
• https://github.com/eslint/eslint/commit/a9a4652b2ce92858d90243e7bb8693f458a14783
• https://github.com/facebookincubator/create-react-app/issues/272
• https://github.com/facebookincubator/create-react-app/issues/274
• https://github.com/yannickcr/eslint-plugin-react/blob/master/lib/rules/jsx-equals-spacing.js#L18
• https://github.com/jfmengels/eslint-rule-documentation/blob/master/contributing.md
• https://github.com/Gillespie59/eslint-plugin-angular
• http://eslint.org/docs/developer-guide/working-with-rules
• http://eslint.org/blog/2016/07/eslint-new-rule-format
• http://eslint.org/docs/developer-guide/working-with-rules-new
• https://github.com/buildo/eslint-plugin-no-loops (had no fixing, was no loops in general)
• https://github.com/airbnb/javascript#iterators--nope
• https://www.paypal-engineering.com/2014/12/12/maintaining-javascript-code-quality-with-eslint/
• https://babeljs.io/repl/
• http://eslint.org/docs/rules/

known bugs

• properly parse Object.keys(apples).forEach(apple => delete apples[apple]) because it replaces apple with new var, but not apples so it leaves the s
• nested loops, will do this soon
• truncating body at some point

jsperf

var testData = [];
for (var i = 0; i < 100; i++) {
  testData.push(i);
}
 
// forEach
var res = 0;
testData.forEach(function(x) {
  res += x;
});
 
// for
var res = 0;
for (var i = 0; i < testData.length; i++) {
  res += testData[i];
}
 
// for optimized
var res = 0;
for (var i = 0, len = testData.length; i < len; i++) {
  res += testData[i];
}
 
// reduce
var res = testData.reduce(function(sum, x) {
  return sum + x;
}, 0);
 
 
// while
var res = 0;
var i = testData.length;
while (i--) {
    res += testData[i];
}
 
// for in
var res = 0;
for (var data in testData) {
  res += testData[i];
}
 
 
// for of
var res = 0;
for (var data of testData) {
  res += testData[i];
}
 
 
// for of babel
var res = 0;
var _iteratorNormalCompletion = true;
var _didIteratorError = false;
var _iteratorError = undefined;
 
try {
  for (var _iterator = testData[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
    var value = _step.value;
 
    console.log(value);
  }
} catch (err) {
  _didIteratorError = true;
  _iteratorError = err;
} finally {
  try {
    res += testData[i];
    if (!_iteratorNormalCompletion && _iterator.return) {
      _iterator.return();
    }
  } finally {
    if (_didIteratorError) {
      throw _iteratorError;
    }
  }
}

@NOTES:

could keep option cache-length but since we are using an object, seems silly to call object.keys 2x

we do not use body.body because

• 1. not always there
• 1. we want to keep curly braces (or lack of from the source

(when it has proper configs and has large files to test, then release 1.0.0)

@TODO:

• add config option for cache-length-inside-loop (var i = 0, len = varName.length; ...)
• recommend let => const rule
• add large js files to test failed fixing & safety
• isDev | isTest helper funcs
• ^ add args in tests for devtest
• comment on readmes
• move todos to issues
• use type in the left of the loop such as const, let, var
• o.hasOwnProperty for for in/of
• make a guide how to make an eslint plugin