eslint/docs/rules/valid-typeof.md

   1 # enforce comparing `typeof` expressions against valid strings (valid-typeof)
   2
   3 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.
   4
   5 ## Rule Details
   6
   7 This rule enforces comparing `typeof` expressions to valid string literals.
   8
   9 ## Options
  10
  11 This rule has an object option:
  12
  13 * `"requireStringLiterals": true` requires `typeof` expressions to only be compared to string literals or other `typeof` expressions, and disallows comparisons to any other value.
  14
  15 Examples of **incorrect** code for this rule:
  16
  17 ```js
  18 /*eslint valid-typeof: "error"*/
  19
  20 typeof foo === "strnig"
  21 typeof foo == "undefimed"
  22 typeof bar != "nunber"
  23 typeof bar !== "fucntion"
  24 ```
  25
  26 Examples of **correct** code for this rule:
  27
  28 ```js
  29 /*eslint valid-typeof: "error"*/
  30
  31 typeof foo === "string"
  32 typeof bar == "undefined"
  33 typeof foo === baz
  34 typeof bar === typeof qux
  35 ```
  36
  37 Examples of **incorrect** code with the `{ "requireStringLiterals": true }` option:
  38
  39 ```js
  40 typeof foo === undefined
  41 typeof bar == Object
  42 typeof baz === "strnig"
  43 typeof qux === "some invalid type"
  44 typeof baz === anotherVariable
  45 typeof foo == 5
  46 ```
  47
  48 Examples of **correct** code with the `{ "requireStringLiterals": true }` option:
  49
  50 ```js
  51 typeof foo === "undefined"
  52 typeof bar == "object"
  53 typeof baz === "string"
  54 typeof bar === typeof qux
  55 ```
  56
  57 ## When Not To Use It
  58
  59 You may want to turn this rule off if you will be using the `typeof` operator on host objects.
  60
  61 ## Further Reading
  62
  63 * [MDN: `typeof` documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof)