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eb39fafa4f
includes a (minimal) working wrapper Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
112 lines
2.2 KiB
Markdown
112 lines
2.2 KiB
Markdown
# Disallow Initializing to undefined (no-undef-init)
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In JavaScript, a variable that is declared and not initialized to any value automatically gets the value of `undefined`. For example:
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```js
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var foo;
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console.log(foo === undefined); // true
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```
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It's therefore unnecessary to initialize a variable to `undefined`, such as:
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```js
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var foo = undefined;
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```
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It's considered a best practice to avoid initializing variables to `undefined`.
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## Rule Details
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This rule aims to eliminate variable declarations that initialize to `undefined`.
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Examples of **incorrect** code for this rule:
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```js
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/*eslint no-undef-init: "error"*/
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/*eslint-env es6*/
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var foo = undefined;
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let bar = undefined;
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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-undef-init: "error"*/
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/*eslint-env es6*/
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var foo;
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let bar;
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const baz = undefined;
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```
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## When Not To Use It
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There is one situation where initializing to `undefined` behaves differently than omitting the initialization, and that's when a `var` declaration occurs inside of a loop. For example:
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Example of **incorrect** code for this rule:
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```js
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for (i = 0; i < 10; i++) {
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var x = undefined;
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console.log(x);
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x = i;
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}
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```
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In this case, the `var x` is hoisted out of the loop, effectively creating:
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```js
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var x;
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for (i = 0; i < 10; i++) {
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x = undefined;
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console.log(x);
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x = i;
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}
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```
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If you were to remove the initialization, then the behavior of the loop changes:
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```js
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for (i = 0; i < 10; i++) {
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var x;
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console.log(x);
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x = i;
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}
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```
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This code is equivalent to:
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```js
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var x;
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for (i = 0; i < 10; i++) {
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console.log(x);
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x = i;
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}
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```
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This produces a different outcome than defining `var x = undefined` in the loop, as `x` is no longer reset to `undefined` each time through the loop.
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If you're using such an initialization inside of a loop, then you should disable this rule.
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Example of **correct** code for this rule, because it is disabled on a specific line:
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```js
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/*eslint no-undef-init: "error"*/
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for (i = 0; i < 10; i++) {
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var x = undefined; // eslint-disable-line no-undef-init
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console.log(x);
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x = i;
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}
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```
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## Related Rules
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* [no-undefined](no-undefined.md)
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* [no-void](no-void.md)
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