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

   1 ---
   2 title: guard-for-in
   3 layout: doc
   4 rule_type: suggestion
   5 related_rules:
   6 - no-prototype-builtins
   7 further_reading:
   8 - https://javascriptweblog.wordpress.com/2011/01/04/exploring-javascript-for-in-loops/
   9 - https://2ality.com/2012/01/objects-as-maps.html
  10 ---
  11
  12
  13 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.
  14
  15 ```js
  16 for (key in foo) {
  17     doSomething(key);
  18 }
  19 ```
  20
  21 Note that simply checking `foo.hasOwnProperty(key)` is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins).
  22
  23 ## Rule Details
  24
  25 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.
  26
  27 Examples of **incorrect** code for this rule:
  28
  29 ::: incorrect
  30
  31 ```js
  32 /*eslint guard-for-in: "error"*/
  33
  34 for (key in foo) {
  35     doSomething(key);
  36 }
  37 ```
  38
  39 :::
  40
  41 Examples of **correct** code for this rule:
  42
  43 ::: correct
  44
  45 ```js
  46 /*eslint guard-for-in: "error"*/
  47
  48 for (key in foo) {
  49     if (Object.prototype.hasOwnProperty.call(foo, key)) {
  50         doSomething(key);
  51     }
  52 }
  53
  54 for (key in foo) {
  55     if ({}.hasOwnProperty.call(foo, key)) {
  56         doSomething(key);
  57     }
  58 }
  59 ```
  60
  61 :::