eslint/docs/src/rules/valid-typeof.md

   1 ---
   2 title: valid-typeof
   3 layout: doc
   4 rule_type: problem
   5 further_reading:
   6 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof
   7 ---
   8
   9
  10
  11
  12
  13 For a vast majority of use cases, the result of the `typeof` operator is one of the following string literals: `"undefined"`, `"object"`, `"boolean"`, `"number"`, `"string"`, `"function"`, `"symbol"`, and `"bigint"`. It is usually a typing mistake to compare the result of a `typeof` operator to other string literals.
  14
  15 ## Rule Details
  16
  17 This rule enforces comparing `typeof` expressions to valid string literals.
  18
  19 ## Options
  20
  21 This rule has an object option:
  22
  23 * `"requireStringLiterals": true` requires `typeof` expressions to only be compared to string literals or other `typeof` expressions, and disallows comparisons to any other value.
  24
  25 Examples of **incorrect** code for this rule:
  26
  27 ::: incorrect
  28
  29 ```js
  30 /*eslint valid-typeof: "error"*/
  31
  32 typeof foo === "strnig"
  33 typeof foo == "undefimed"
  34 typeof bar != "nunber"
  35 typeof bar !== "fucntion"
  36 ```
  37
  38 :::
  39
  40 Examples of **correct** code for this rule:
  41
  42 ::: correct
  43
  44 ```js
  45 /*eslint valid-typeof: "error"*/
  46
  47 typeof foo === "string"
  48 typeof bar == "undefined"
  49 typeof foo === baz
  50 typeof bar === typeof qux
  51 ```
  52
  53 :::
  54
  55 Examples of **incorrect** code with the `{ "requireStringLiterals": true }` option:
  56
  57 ::: incorrect
  58
  59 ```js
  60 /*eslint valid-typeof: ["error", { "requireStringLiterals": true }]*/
  61
  62 typeof foo === undefined
  63 typeof bar == Object
  64 typeof baz === "strnig"
  65 typeof qux === "some invalid type"
  66 typeof baz === anotherVariable
  67 typeof foo == 5
  68 ```
  69
  70 :::
  71
  72 Examples of **correct** code with the `{ "requireStringLiterals": true }` option:
  73
  74 ::: correct
  75
  76 ```js
  77 /*eslint valid-typeof: ["error", { "requireStringLiterals": true }]*/
  78
  79 typeof foo === "undefined"
  80 typeof bar == "object"
  81 typeof baz === "string"
  82 typeof bar === typeof qux
  83 ```
  84
  85 :::
  86
  87 ## When Not To Use It
  88
  89 You may want to turn this rule off if you will be using the `typeof` operator on host objects.