disallow variable declarations from shadowing variables declared in the outer scope (no-shadow)

禁止变量声明覆盖外层作用域的变量 (no-shadow)

Shadowing is the process by which a local variable shares the same name as a variable in its containing scope. For example:

覆盖是指在同一作用域里局部变量和全局变量同名,比如:

var a = 3;
function b() {
    var a = 10;
}

In this case, the variable a inside of b() is shadowing the variable a in the global scope. This can cause confusion while reading the code and it’s impossible to access the global variable.

在这种情况中,b() 作用域中的 a 覆盖了全局环境中的 a。这会混淆读者并且在 b中不能获取全局变量。

Rule Details

This rule aims to eliminate shadowed variable declarations.

此规则旨在消除变量声明覆盖。

Examples of incorrect code for this rule:

错误 代码示例:

/*eslint no-shadow: "error"*/
/*eslint-env es6*/

var a = 3;
function b() {
    var a = 10;
}

var b = function () {
    var a = 10;
}

function b(a) {
    a = 10;
}
b(a);

if (true) {
    let a = 5;
}

Options

This rule takes one option, an object, with properties "builtinGlobals", "hoist" and "allow".

对象配置项,包含属性 "builtinGlobals""hoist""allow".

{
    "no-shadow": ["error", { "builtinGlobals": false, "hoist": "functions", "allow": [] }]
}

builtinGlobals

The builtinGlobals option is false by default. If it is true, the rule prevents shadowing of built-in global variables: Object, Array, Number, and so on.

默认值是 false,如果 builtinGlobalstrue,会检测内置对象如 ObjectArrayNumber 等等。

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

选项 {"builtinGlobals": true}错误 代码示例:

/*eslint no-shadow: ["error", { "builtinGlobals": true }]*/

function foo() {
    var Object = 0;
}

hoist

The hoist option has three settings:

此配置项有三个值:

hoist: functions

Examples of incorrect code for the default { "hoist": "functions" } option:

默认选项 { "hoist": "functions" }错误 代码示例:

/*eslint no-shadow: ["error", { "hoist": "functions" }]*/
/*eslint-env es6*/

if (true) {
    let b = 6;
}

function b() {}

Although let b in the if statement is before the function declaration in the outer scope, it is incorrect.

虽然 if 语句中的 let bfunction声明之前,该示例是正确的。

Examples of correct code for the default { "hoist": "functions" } option:

默认选项 { "hoist": "functions" }正确 代码示例:

/*eslint no-shadow: ["error", { "hoist": "functions" }]*/
/*eslint-env es6*/

if (true) {
    let a = 3;
}

let a = 5;

Because let a in the if statement is before the variable declaration in the outer scope, it is correct.

因为在 if 语句中 let a 在外层作用域声明语句之前,所以是正确的。

hoist: all

Examples of incorrect code for the { "hoist": "all" } option:

选项 { "hoist": "all" }错误 代码示例:

/*eslint no-shadow: ["error", { "hoist": "all" }]*/
/*eslint-env es6*/

if (true) {
    let a = 3;
    let b = 6;
}

let a = 5;
function b() {}

hoist: never

Examples of correct code for the { "hoist": "never" } option:

选项 { "hoist": "never" }正确 代码示例:

/*eslint no-shadow: ["error", { "hoist": "never" }]*/
/*eslint-env es6*/

if (true) {
    let a = 3;
    let b = 6;
}

let a = 5;
function b() {}

Because let a and let b in the if statement are before the declarations in the outer scope, they are correct.

因为在 if 语句中 let alet b 在外层作用域声明语句之前,所以是正确的。

allow

The allow option is an array of identifier names for which shadowing is allowed. For example, "resolve", "reject", "done", "cb".

allow 选项是个标识符名称的数组,以允许他们被重写。例如:"resolve""reject""done""cb"

Examples of correct code for the { "allow": ["done"] } option:

选项 { "allow": ["done"] }正确 代码示例:

/*eslint no-shadow: ["error", { "allow": ["done"] }]*/
/*eslint-env es6*/

import async from 'async';

function foo(done) {
  async.map([1, 2], function (e, done) {
    done(null, e * 2)
  }, done);
}

foo(function (err, result) {
  console.log({ err, result });
});

Further Reading

Version

This rule was introduced in ESLint 0.0.9.

该规则在 ESLint 0.0.9 中被引入。

Resources


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