projects / pve-eslint.git / blame
| Commit | Line | Data |
|---|---|---|
| eb39fafa DC |
1 | # Disallow Labeled Statements (no-labels) |
| 2 | ||
| 3 | Labeled statements in JavaScript are used in conjunction with `break` and `continue` to control flow around multiple loops. For example: | |
| 4 | ||
| 5 | ```js | |
| 6 | outer: | |
| 7 | while (true) { | |
| 8 | ||
| 9 | while (true) { | |
| 10 | break outer; | |
| 11 | } | |
| 12 | } | |
| 13 | ``` | |
| 14 | ||
| 15 | The `break outer` statement ensures that this code will not result in an infinite loop because control is returned to the next statement after the `outer` label was applied. If this statement was changed to be just `break`, control would flow back to the outer `while` statement and an infinite loop would result. | |
| 16 | ||
| 17 | While convenient in some cases, labels tend to be used only rarely and are frowned upon by some as a remedial form of flow control that is more error prone and harder to understand. | |
| 18 | ||
| 19 | ## Rule Details | |
| 20 | ||
| 21 | This rule aims to eliminate the use of labeled statements in JavaScript. It will warn whenever a labeled statement is encountered and whenever `break` or `continue` are used with a label. | |
| 22 | ||
| 23 | Examples of **incorrect** code for this rule: | |
| 24 | ||
| 25 | ```js | |
| 26 | /*eslint no-labels: "error"*/ | |
| 27 | ||
| 28 | label: | |
| 29 | while(true) { | |
| 30 | // ... | |
| 31 | } | |
| 32 | ||
| 33 | label: | |
| 34 | while(true) { | |
| 35 | break label; | |
| 36 | } | |
| 37 | ||
| 38 | label: | |
| 39 | while(true) { | |
| 40 | continue label; | |
| 41 | } | |
| 42 | ||
| 43 | label: | |
| 44 | switch (a) { | |
| 45 | case 0: | |
| 46 | break label; | |
| 47 | } | |
| 48 | ||
| 49 | label: | |
| 50 | { | |
| 51 | break label; | |
| 52 | } | |
| 53 | ||
| 54 | label: | |
| 55 | if (a) { | |
| 56 | break label; | |
| 57 | } | |
| 58 | ``` | |
| 59 | ||
| 60 | Examples of **correct** code for this rule: | |
| 61 | ||
| 62 | ```js | |
| 63 | /*eslint no-labels: "error"*/ | |
| 64 | ||
| 65 | var f = { | |
| 66 | label: "foo" | |
| 67 | }; | |
| 68 | ||
| 69 | while (true) { | |
| 70 | break; | |
| 71 | } | |
| 72 | ||
| 73 | while (true) { | |
| 74 | continue; | |
| 75 | } | |
| 76 | ``` | |
| 77 | ||
| 78 | ## Options | |
| 79 | ||
| 80 | The options allow labels with loop or switch statements: | |
| 81 | ||
| 82 | * `"allowLoop"` (`boolean`, default is `false`) - If this option was set `true`, this rule ignores labels which are sticking to loop statements. | |
| 83 | * `"allowSwitch"` (`boolean`, default is `false`) - If this option was set `true`, this rule ignores labels which are sticking to switch statements. | |
| 84 | ||
| 85 | Actually labeled statements in JavaScript can be used with other than loop and switch statements. | |
| 86 | However, this way is ultra rare, not well-known, so this would be confusing developers. | |
| 87 | ||
| 88 | ### allowLoop | |
| 89 | ||
| 90 | Examples of **correct** code for the `{ "allowLoop": true }` option: | |
| 91 | ||
| 92 | ```js | |
| 93 | /*eslint no-labels: ["error", { "allowLoop": true }]*/ | |
| 94 | ||
| 95 | label: | |
| 96 | while (true) { | |
| 97 | break label; | |
| 98 | } | |
| 99 | ``` | |
| 100 | ||
| 101 | ### allowSwitch | |
| 102 | ||
| 103 | Examples of **correct** code for the `{ "allowSwitch": true }` option: | |
| 104 | ||
| 105 | ```js | |
| 106 | /*eslint no-labels: ["error", { "allowSwitch": true }]*/ | |
| 107 | ||
| 108 | label: | |
| 109 | switch (a) { | |
| 110 | case 0: | |
| 111 | break label; | |
| 112 | } | |
| 113 | ``` | |
| 114 | ||
| 115 | ## When Not To Use It | |
| 116 | ||
| 117 | If you need to use labeled statements everywhere, then you can safely disable this rule. | |
| 118 | ||
| 119 | ## Related Rules | |
| 120 | ||
| 121 | * [no-extra-label](./no-extra-label.md) | |
| 122 | * [no-label-var](./no-label-var.md) | |
| 123 | * [no-unused-labels](./no-unused-labels.md) |