eslint/docs/rules/guard-for-in.md

   1 # Require Guarding for-in (guard-for-in)
   2
   3 Looping over objects with a `for in` loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.
   4
   5 ```js
   6 for (key in foo) {
   7     doSomething(key);
   8 }
   9 ```
  10
  11 Note that simply checking `foo.hasOwnProperty(key)` is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins.md).
  12
  13 ## Rule Details
  14
  15 This rule is aimed at preventing unexpected behavior that could arise from using a `for in` loop without filtering the results in the loop. As such, it will warn when `for in` loops do not filter their results with an `if` statement.
  16
  17 Examples of **incorrect** code for this rule:
  18
  19 ```js
  20 /*eslint guard-for-in: "error"*/
  21
  22 for (key in foo) {
  23     doSomething(key);
  24 }
  25 ```
  26
  27 Examples of **correct** code for this rule:
  28
  29 ```js
  30 /*eslint guard-for-in: "error"*/
  31
  32 for (key in foo) {
  33     if (Object.prototype.hasOwnProperty.call(foo, key)) {
  34         doSomething(key);
  35     }
  36 }
  37
  38 for (key in foo) {
  39     if ({}.hasOwnProperty.call(foo, key)) {
  40         doSomething(key);
  41     }
  42 }
  43 ```
  44
  45 ## Related Rules
  46
  47 * [no-prototype-builtins](no-prototype-builtins.md)
  48
  49 ## Further Reading
  50
  51 * [Exploring JavaScript for-in loops](https://javascriptweblog.wordpress.com/2011/01/04/exploring-javascript-for-in-loops/)
  52 * [The pitfalls of using objects as maps in JavaScript](http://2ality.com/2012/01/objects-as-maps.html)