no-undefined

禁止使用 undefined 作为标识符

JavaScript 中的 undefined 变量实际上是全局对象的一个属性。因此，在 ECMAScript 3 中，可以覆盖 undefined 的值。虽然 ECMAScript 5 不允许覆盖 undefined，但仍然可以覆盖 undefined，例如：

¥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:

function doSomething(data) {
    var undefined = "hi";

    // doesn't do what you think it does
    if (data === undefined) {
        // ...
    }

}
1
2
3
4
5
6
7
8
9

因为 undefined 可以被覆盖或隐藏，读取 undefined 可能会给出一个意想不到的值。（null 的情况并非如此，它是一个始终产生相同值的关键字。）为了防止这种情况，你可以避免使用 undefined，这是一些样式指南建议的内容，也是此规则强制执行的内容。这些风格指南还建议：

¥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:

应该是 undefined 的变量只是未初始化。（所有未初始化的变量在 JavaScript 中自动获取 undefined 的值。）

¥Variables that should be undefined are simply left uninitialized. (All uninitialized variables automatically get the value of undefined in JavaScript.)
检查值是否为 undefined 应使用 typeof 完成。

¥Checking if a value is undefined should be done with typeof.
如有必要，使用 void 运算符生成 undefined 的值。

¥Using the void operator to generate the value of undefined if necessary.

作为替代方案，你可以使用 no-global-assign 和 no-shadow-restricted-names 规则来防止 undefined 被遮蔽或分配不同的值。这确保了 undefined 将始终保持其原始预期值。

¥As an alternative, you can use the no-global-assign and no-shadow-restricted-names rules to prevent undefined from being shadowed or assigned a different value. This ensures that undefined will always hold its original, expected value.

规则详情

¥Rule Details

此规则旨在消除对 undefined 的使用，因此，每当使用它时都会生成警告。

¥This rule aims to eliminate the use of undefined, and as such, generates a warning whenever it is used.

此规则的错误代码示例：

¥Examples of incorrect code for this rule:

在线运行

/*eslint no-undefined: "error"*/

var foo = undefined;

var undefined = "foo";

if (foo === undefined) {
    // ...
}

function baz(undefined) {
    // ...
}

bar(undefined, "lorem");
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

此规则的正确代码示例：

¥Examples of correct code for this rule:

在线运行

/*eslint no-undefined: "error"*/

var foo = void 0;

var Undefined = "foo";

if (typeof foo === "undefined") {
    // ...
}

global.undefined = "foo";

bar(void 0, "lorem");
1
2
3
4
5
6
7
8
9
10
11
12
13

何时不使用

¥When Not To Use It

如果你想允许在你的代码中使用 undefined，那么你可以安全地关闭此规则。

¥If you want to allow the use of undefined in your code, then you can safely turn this rule off.

版本

此规则是在 ESLint v0.7.1 中引入。

进阶读物

undefined - JavaScript | MDN
web.nodejs.cn

Understanding JavaScript’s ‘undefined’
javascriptweblog.wordpress.com

Annotated ES5
es5.github.io

no-undefined

规则详情

何时不使用

相关规则

版本

进阶读物

资源