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includes a (minimal) working wrapper Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
80 lines
2.7 KiB
Markdown
80 lines
2.7 KiB
Markdown
# Disallow Use of `undefined` Variable (no-undefined)
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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:
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```js
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function doSomething(data) {
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var undefined = "hi";
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// doesn't do what you think it does
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if (data === undefined) {
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// ...
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}
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}
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```
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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:
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* Variables that should be `undefined` are simply left uninitialized. (All uninitialized variables automatically get the value of `undefined` in JavaScript.)
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* Checking if a value is `undefined` should be done with `typeof`.
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* Using the `void` operator to generate the value of `undefined` if necessary.
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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.
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## Rule Details
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This rule aims to eliminate the use of `undefined`, and as such, generates a warning whenever it is used.
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Examples of **incorrect** code for this rule:
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```js
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/*eslint no-undefined: "error"*/
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var foo = undefined;
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var undefined = "foo";
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if (foo === undefined) {
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// ...
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}
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function foo(undefined) {
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// ...
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}
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```
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Examples of **correct** code for this rule:
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```js
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/*eslint no-undefined: "error"*/
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var foo = void 0;
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var Undefined = "foo";
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if (typeof foo === "undefined") {
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// ...
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}
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global.undefined = "foo";
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```
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## When Not To Use It
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If you want to allow the use of `undefined` in your code, then you can safely turn this rule off.
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## Further Reading
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* [undefined - JavaScript \| MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined)
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* [Understanding JavaScript’s ‘undefined’ \| JavaScript, JavaScript...](https://javascriptweblog.wordpress.com/2010/08/16/understanding-undefined-and-preventing-referenceerrors/)
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* [ECMA262 edition 5.1 §15.1.1.3: undefined](https://es5.github.io/#x15.1.1.3)
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## Related Rules
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* [no-undef-init](no-undef-init.md)
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* [no-void](no-void.md)
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* [no-shadow-restricted-names](no-shadow-restricted-names.md)
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* [no-global-assign](no-global-assign.md)
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