no-undef-init

不允许将变量初始化为 undefined

🔧

此规则报告的一些问题可通过 --fix 命令行 选项自动修复

❄️

此规则目前已 冻结，不接受功能请求。

在 JavaScript 中，已声明但未初始化为任何值的变量会自动获取 undefined 的值。例如：

¥In JavaScript, a variable that is declared and not initialized to any value automatically gets the value of undefined. For example:

var foo;

console.log(foo === undefined);     // true

因此不必将变量初始化为 undefined，例如：

¥It’s therefore unnecessary to initialize a variable to undefined, such as:

var foo = undefined;

避免将变量初始化为 undefined 被认为是最佳实践。

¥It’s considered a best practice to avoid initializing variables to undefined.

规则详情

¥Rule Details

此规则旨在消除初始化为 undefined 的 var 和 let 变量声明。

¥This rule aims to eliminate var and let variable declarations that initialize to undefined.

此规则的错误代码示例：

¥Examples of incorrect code for this rule:

在线运行 
/*eslint no-undef-init: "error"*/

var foo = undefined;
let bar = undefined;

此规则的正确代码示例：

¥Examples of correct code for this rule:

在线运行 
/*eslint no-undef-init: "error"*/

var foo;
let bar;

请注意，此规则不检查 const 声明、解构模式、函数参数和类字段。

¥Please note that this rule does not check const declarations, destructuring patterns, function parameters, and class fields.

此规则的附加正确代码示例：

¥Examples of additional correct code for this rule:

在线运行 
/*eslint no-undef-init: "error"*/

const foo = undefined;

let { bar = undefined } = baz;

[quux = undefined] = quuux;

(foo = undefined) => {};

class Foo {
    bar = undefined;
}

何时不使用

¥When Not To Use It

在某些情况下，初始化为 undefined 的行为与省略初始化的行为不同。

¥There are situations where initializing to undefined behaves differently than omitting the initialization.

其中一种情况是 var 声明出现在循环内部。例如：

¥One such case is when a var declaration occurs inside of a loop. For example:

此规则的错误代码示例：

¥Example of incorrect code for this rule:

在线运行 
/*eslint no-undef-init: "error"*/

for (i = 0; i < 10; i++) {
    var x = undefined;
    console.log(x);
    x = i;
}

在这种情况下，var x 被吊出循环，有效地创建：

¥In this case, the var x is hoisted out of the loop, effectively creating:

var x;

for (i = 0; i < 10; i++) {
    x = undefined;
    console.log(x);
    x = i;
}

如果要删除初始化，则循环的行为会发生变化：

¥If you were to remove the initialization, then the behavior of the loop changes:

for (i = 0; i < 10; i++) {
    var x;
    console.log(x);
    x = i;
}

这段代码相当于：

¥This code is equivalent to:

var x;

for (i = 0; i < 10; i++) {
    console.log(x);
    x = i;
}

这会产生与在循环中定义 var x = undefined 不同的结果，因为 x 不再每次通过循环都重置为 undefined。

¥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.

如果你在循环内使用这样的初始化，那么你应该禁用此规则。

¥If you’re using such an initialization inside of a loop, then you should disable this rule.

此规则的正确代码示例，因为它在特定行上被禁用：

¥Example of correct code for this rule, because it is disabled on a specific line:

在线运行 
/*eslint no-undef-init: "error"*/

for (i = 0; i < 10; i++) {
    var x = undefined; // eslint-disable-line no-undef-init
    console.log(x);
    x = i;
}

另一种情况是使用 var 重新声明变量。例如：

¥Another such case is when a variable is redeclared using var. For example:

function foo() {
    var x = 1;
    console.log(x); // output: 1
    
    var x;
    console.log(x); // output: 1

    var x = undefined;
    console.log(x); // output: undefined
}

foo();

在这种情况下，你可以通过将其更改为赋值 (x = undefined;) 来避免重新声明，或者在特定行上使用 eslint disable 注释。

¥In this case, you can avoid redeclaration by changing it to an assignment (x = undefined;), or use an eslint disable comment on a specific line.

版本

此规则是在 ESLint v0.0.6 中引入。

资源