Disallow Initializing to undefined (no-undef-init)

不允许初始化变量值为 undefined (no-undef-init)

The --fix option on the command line can automatically fix some of the problems reported by this rule.

命令行中的 --fix 选项可以自动修复一些该规则报告的问题。

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

在 JavaScript 中，声明一个变量但未初始化，变量会自动获得 undefined 作为初始值。比如：

var foo;

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

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

因此，初始化变量值为 undefined 是多余的，如：

var foo = undefined;

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

最好的做法是避免初始化变量值为 undefined。

Rule Details

This rule aims to eliminate variable declarations that initialize to undefined.

此规则旨在限制变量声明后被初始化为 undefined。

Examples of incorrect code for this rule:

错误 代码示例：

/*eslint no-undef-init: "error"*/
/*eslint-env es6*/

var foo = undefined;
let bar = undefined;

Examples of correct code for this rule:

正确 代码示例：

/*eslint no-undef-init: "error"*/
/*eslint-env es6*/

var foo;
let bar;
const baz = undefined;

When Not To Use It

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:

当在一个循环中用 var 声明一个变量的时候，这种情况下给变量赋值为 undefined 与省略初始化的结果不同。比如：

Example of incorrect code for this rule:

错误 代码示例：

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

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

在这种情况下，var x 被提升到循环外。

var x;

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

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

如果删除初始化语句，循环语句的行为会改变：

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

This code is equivalent to:

此代码等价于下面代码：

var x;

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

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.

于在循环语句中使用 var x = undefined 相比，这样产生了一个不同结果 ，因为每次循环后 x 不在赋值为 undefined。

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

如果你有在循环语句中初始化变量，你应该禁用此规则。

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

正确 代码示例，因为它在特定的行上被禁用。

/*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;
}

Version

This rule was introduced in ESLint 0.0.6.

该规则在 ESLint 0.0.6 中被引入。

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