projects / pve-eslint.git / blame
| Commit | Line | Data |
|---|---|---|
| eb39fafa DC |
1 | # Disallow use of the void operator. (no-void) |
| 2 | ||
| 3 | The `void` operator takes an operand and returns `undefined`: `void expression` will evaluate `expression` and return `undefined`. It can be used to ignore any side effects `expression` may produce: | |
| 4 | ||
| 5 | The common case of using `void` operator is to get a "pure" `undefined` value as prior to ES5 the `undefined` variable was mutable: | |
| 6 | ||
| 7 | ```js | |
| 8 | // will always return undefined | |
| 9 | (function(){ | |
| 10 | return void 0; | |
| 11 | })(); | |
| 12 | ||
| 13 | // will return 1 in ES3 and undefined in ES5+ | |
| 14 | (function(){ | |
| 15 | undefined = 1; | |
| 16 | return undefined; | |
| 17 | })(); | |
| 18 | ||
| 19 | // will throw TypeError in ES5+ | |
| 20 | (function(){ | |
| 21 | 'use strict'; | |
| 22 | undefined = 1; | |
| 23 | })(); | |
| 24 | ``` | |
| 25 | ||
| 26 | Another common case is to minify code as `void 0` is shorter than `undefined`: | |
| 27 | ||
| 28 | ```js | |
| 29 | foo = void 0; | |
| 30 | foo = undefined; | |
| 31 | ``` | |
| 32 | ||
| 33 | When used with IIFE (immediately-invoked function expression), `void` can be used to force the function keyword to be treated as an expression instead of a declaration: | |
| 34 | ||
| 35 | ```js | |
| 36 | var foo = 1; | |
| 37 | void function(){ foo = 1; }() // will assign foo a value of 1 | |
| 38 | +function(){ foo = 1; }() // same as above | |
| 39 | ``` | |
| 40 | ||
| 34eeec05 | 41 | ```js |
| eb39fafa DC |
42 | function(){ foo = 1; }() // will throw SyntaxError |
| 43 | ``` | |
| 44 | ||
| 45 | Some code styles prohibit `void` operator, marking it as non-obvious and hard to read. | |
| 46 | ||
| 47 | ## Rule Details | |
| 48 | ||
| 49 | This rule aims to eliminate use of void operator. | |
| 50 | ||
| 51 | Examples of **incorrect** code for this rule: | |
| 52 | ||
| 53 | ```js | |
| 54 | /*eslint no-void: "error"*/ | |
| 55 | ||
| 56 | void foo | |
| 57 | void someFunction(); | |
| 58 | ||
| 59 | var foo = void bar(); | |
| 60 | function baz() { | |
| 61 | return void 0; | |
| 62 | } | |
| 63 | ``` | |
| 64 | ||
| 65 | ## Options | |
| 66 | ||
| 67 | This rule has an object option: | |
| 68 | ||
| 69 | * `allowAsStatement` set to `true` allows the void operator to be used as a statement (Default `false`). | |
| 70 | ||
| 71 | ### allowAsStatement | |
| 72 | ||
| 73 | When `allowAsStatement` is set to true, the rule will not error on cases that the void operator is used as a statement, i.e. when it's not used in an expression position, like in a variable assignment or a function return. | |
| 74 | ||
| 75 | Examples of **incorrect** code for `{ "allowAsStatement": true }`: | |
| 76 | ||
| 77 | ```js | |
| 78 | /*eslint no-void: ["error", { "allowAsStatement": true }]*/ | |
| 79 | ||
| 80 | var foo = void bar(); | |
| 81 | function baz() { | |
| 82 | return void 0; | |
| 83 | } | |
| 84 | ``` | |
| 85 | ||
| 86 | Examples of **correct** code for `{ "allowAsStatement": true }`: | |
| 87 | ||
| 88 | ```js | |
| 89 | /*eslint no-void: ["error", { "allowAsStatement": true }]*/ | |
| 90 | ||
| 91 | void foo; | |
| 92 | void someFunction(); | |
| 93 | ``` | |
| 94 | ||
| 95 | ## When Not To Use It | |
| 96 | ||
| 97 | If you intentionally use the `void` operator then you can disable this rule. | |
| 98 | ||
| 99 | ## Further Reading | |
| 100 | ||
| 101 | * [Mozilla Developer Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void) | |
| 102 | * [Bad Parts: Appendix B - JavaScript: The Good Parts by Douglas Crockford](https://oreilly.com/javascript/excerpts/javascript-good-parts/bad-parts.html) | |
| 103 | ||
| 104 | ## Related Rules | |
| 105 | ||
| 106 | * [no-undef-init](no-undef-init.md) | |
| 107 | * [no-undefined](no-undefined.md) |