projects / pve-eslint.git / blame
| Commit | Line | Data |
|---|---|---|
| eb39fafa DC |
1 | # Disallow Use of `undefined` Variable (no-undefined) |
| 2 | ||
| 3 | The `undefined` variable in JavaScript is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of `undefined`. While ECMAScript 5 disallows overwriting `undefined`, it's still possible to shadow `undefined`, such as: | |
| 4 | ||
| 5 | ```js | |
| 6 | function doSomething(data) { | |
| 7 | var undefined = "hi"; | |
| 8 | ||
| 9 | // doesn't do what you think it does | |
| 10 | if (data === undefined) { | |
| 11 | // ... | |
| 12 | } | |
| 13 | ||
| 14 | } | |
| 15 | ``` | |
| 16 | ||
| 17 | Because `undefined` can be overwritten or shadowed, reading `undefined` can give an unexpected value. (This is not the case for `null`, which is a keyword that always produces the same value.) To guard against this, you can avoid all uses of `undefined`, which is what some style guides recommend and what this rule enforces. Those style guides then also recommend: | |
| 18 | ||
| 19 | * Variables that should be `undefined` are simply left uninitialized. (All uninitialized variables automatically get the value of `undefined` in JavaScript.) | |
| 20 | * Checking if a value is `undefined` should be done with `typeof`. | |
| 21 | * Using the `void` operator to generate the value of `undefined` if necessary. | |
| 22 | ||
| 23 | As an alternative, you can use the [no-global-assign](no-global-assign.md) and [no-shadow-restricted-names](no-shadow-restricted-names.md) rules to prevent `undefined` from being shadowed or assigned a different value. This ensures that `undefined` will always hold its original, expected value. | |
| 24 | ||
| eb39fafa DC |
25 | ## Rule Details |
| 26 | ||
| 27 | This rule aims to eliminate the use of `undefined`, and as such, generates a warning whenever it is used. | |
| 28 | ||
| 29 | Examples of **incorrect** code for this rule: | |
| 30 | ||
| 31 | ```js | |
| 32 | /*eslint no-undefined: "error"*/ | |
| 33 | ||
| 34 | var foo = undefined; | |
| 35 | ||
| 36 | var undefined = "foo"; | |
| 37 | ||
| 38 | if (foo === undefined) { | |
| 39 | // ... | |
| 40 | } | |
| 41 | ||
| 42 | function foo(undefined) { | |
| 43 | // ... | |
| 44 | } | |
| 45 | ``` | |
| 46 | ||
| 47 | Examples of **correct** code for this rule: | |
| 48 | ||
| 49 | ```js | |
| 50 | /*eslint no-undefined: "error"*/ | |
| 51 | ||
| 52 | var foo = void 0; | |
| 53 | ||
| 54 | var Undefined = "foo"; | |
| 55 | ||
| 56 | if (typeof foo === "undefined") { | |
| 57 | // ... | |
| 58 | } | |
| 59 | ||
| 60 | global.undefined = "foo"; | |
| 61 | ``` | |
| 62 | ||
| 63 | ## When Not To Use It | |
| 64 | ||
| 65 | If you want to allow the use of `undefined` in your code, then you can safely turn this rule off. | |
| 66 | ||
| 67 | ## Further Reading | |
| 68 | ||
| 69 | * [undefined - JavaScript \| MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined) | |
| 70 | * [Understanding JavaScript’s ‘undefined’ \| JavaScript, JavaScript...](https://javascriptweblog.wordpress.com/2010/08/16/understanding-undefined-and-preventing-referenceerrors/) | |
| 71 | * [ECMA262 edition 5.1 §15.1.1.3: undefined](https://es5.github.io/#x15.1.1.3) | |
| 72 | ||
| 73 | ## Related Rules | |
| 74 | ||
| 75 | * [no-undef-init](no-undef-init.md) | |
| 76 | * [no-void](no-void.md) | |
| 77 | * [no-shadow-restricted-names](no-shadow-restricted-names.md) | |
| 78 | * [no-global-assign](no-global-assign.md) |