Disallow Undeclared Variables (no-undef)

禁用未声明的变量 (no-undef)

The "extends": "eslint:recommended" property in a configuration file enables this rule.

配置文件中的 "extends": "eslint:recommended" 属性启用了此规则。

This rule can help you locate potential ReferenceErrors resulting from misspellings of variable and parameter names, or accidental implicit globals (for example, from forgetting the var keyword in a for loop initializer).

此规则可帮助你定位由变量漏写、参数名漏写和意外的隐式全局变量声明所导致的潜在引用错误（比如，在 for 循环语句中初始化变量忘写 var 关键字）

Rule Details

Any reference to an undeclared variable causes a warning, unless the variable is explicitly mentioned in a /*global ...*/ comment, or specified in the globals key in the configuration file. A common use case for these is if you intentionally use globals that are defined elsewhere (e.g. in a script sourced from HTML).

对任何未声明的变量的引用都会引起一个警告，除非显式地在 /*global ...*/ 注释中指定，或在 globals key in the configuration file 中指定。另一个常见的用例是，你有意使用定义在其他地方的全局变量(例如来自 HTML 的脚本)。

Examples of incorrect code for this rule:

错误 代码示例：

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

var a = someFunction();
b = 10;

Examples of correct code for this rule with global declaration:

有 global 声明时，该规则的 正确 代码示例：

/*global someFunction b:true*/
/*eslint no-undef: "error"*/

var a = someFunction();
b = 10;

The b:true syntax in /*global */ indicates that assignment to b is correct.

/*global */ 中的 b:true 表明对 b 继续赋值是正确的。

Examples of incorrect code for this rule with global declaration:

有 global 声明时，该规则的 错误 代码示例：

/*global b*/
/*eslint no-undef: "error"*/

b = 10;

By default, variables declared in /*global */ are read-only, therefore assignment is incorrect.

默认情况下，/*global */ 中声明的变量是只读的，因此对其进行赋值是错误的。

Options

• typeof set to true will warn for variables used inside typeof check (Default false).
• typeof 设置为 true，将对 typeof 中用到的变量发出警告（默认为false）。

typeof

Examples of correct code for the default { "typeof": false } option:

默认选项 { "typeof": false } 的 正确 代码示例：

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

if (typeof UndefinedIdentifier === "undefined") {
    // do something ...
}

You can use this option if you want to prevent typeof check on a variable which has not been declared.

如果想阻止在 typeof 运算中有未申明的变量导致的警告，可以用此项。

Examples of incorrect code for the { "typeof": true } option:

选项 { "typeof": true } 的 错误 代码示例：

/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Examples of correct code for the { "typeof": true } option with global declaration:

有 global 声明时，选项 { "typeof": true } 的 正确 代码示例：

/*global a*/
/*eslint no-undef: ["error", { "typeof": true }] */

if(typeof a === "string"){}

Environments

For convenience, ESLint provides shortcuts that pre-define global variables exposed by popular libraries and runtime environments. This rule supports these environments, as listed in Specifying Environments. A few examples are given below.

为了方便，ESlint 提供了预定义流行类库和运行时环境暴露的全局变量的快捷方式。该规则支持这些环境，如 指定 Environments 中列出的。使用如下：

browser

Examples of correct code for this rule with browser environment:

browser 环境下的 正确 代码示例：

/*eslint no-undef: "error"*/
/*eslint-env browser*/

setTimeout(function() {
    alert("Hello");
});

Node.js

Examples of correct code for this rule with node environment:

node 环境下的 正确 代码示例：

/*eslint no-undef: "error"*/
/*eslint-env node*/

var fs = require("fs");
module.exports = function() {
    console.log(fs);
};

When Not To Use It

If explicit declaration of global variables is not to your taste.

如果你不需要明确声明全局变量，可以关闭此规则。

Compatibility

This rule provides compatibility with treatment of global variables in JSHint and JSLint.

该规则提供了合适对待全局变量的方法。 它在 JSHint 和JSLint。

Version

This rule was introduced in ESLint 0.0.9.

该规则在 ESLint 0.0.9 被引入。

Resources

• Rule source
• Documentation source