prefer-const

在命令行上的--fix选项可以自动修复一些被这条规则反映的问题。

如果一个变量从不重新分配，使用const声明更好。

const 声明告诉读者，“这个变量永远不会被重新分配，”减少认知负荷并提高可维护性。

规则细节

此规则旨在标记使用let关键字声明的变量，但在初始分配后从未重新分配变量。

此规则的错误代码示例：

/*eslint prefer-const: "error"*/
/*eslint-env es6*/

// it's initialized and never reassigned.
let a = 3;
console.log(a);

let a;
a = 0;
console.log(a);

// `i` is redefined (not reassigned) on each loop step.
for (let i in [1, 2, 3]) {
    console.log(i);
}

// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
    console.log(a);
}

此规则的正确代码示例：

/*eslint prefer-const: "error"*/
/*eslint-env es6*/

// using const.
const a = 0;

// it's never initialized.
let a;
console.log(a);

// it's reassigned after initialized.
let a;
a = 0;
a = 1;
console.log(a);

// it's initialized in a different block from the declaration.
let a;
if (true) {
    a = 0;
}
console.log(a);

// it's initialized at a place that we cannot write a variable declaration.
let a;
if (true) a = 0;
console.log(a);

// `i` gets a new binding each iteration
for (const i in [1, 2, 3]) {
  console.log(i);
}

// `a` gets a new binding each iteration
for (const a of [1, 2, 3]) {
  console.log(a);
}

// `end` is never reassigned, but we cannot separate the declarations without modifying the scope.
for (let i = 0, end = 10; i < end; ++i) {
    console.log(a);
}

// suggest to use `no-var` rule.
var b = 3;
console.log(b);

选项

{
    "prefer-const": ["error", {
        "destructuring": "any",
        "ignoreReadBeforeAssign": false
    }]
}

解构

在解构中解决变量的方式。有2个值：

• "any"（默认） - 如果解构中的任何变量应该是const，则此规则将警告这些变量。

• "all"- 如果解构中的所有变量应该是const，则此规则会警告变量。否则，忽略它们。

不正确的代码为默认{"destructuring": "any"}选项的示例：

/*eslint prefer-const: "error"*/
/*eslint-env es6*/

let {a, b} = obj;    /*error 'b' is never reassigned, use 'const' instead.*/
a = a + 1;

默认{"destructuring": "any"}选项的正确代码示例：

/*eslint prefer-const: "error"*/
/*eslint-env es6*/

// using const.
const {a: a0, b} = obj;
const a = a0 + 1;

// all variables are reassigned.
let {a, b} = obj;
a = a + 1;
b = b + 1;

{"destructuring": "all"}选项的错误代码示例：

/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/

// all of `a` and `b` should be const, so those are warned.
let {a, b} = obj;    /*error 'a' is never reassigned, use 'const' instead.
                             'b' is never reassigned, use 'const' instead.*/

选项的正确代码示例{"destructuring": "all"}：

/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/

// 'b' is never reassigned, but all of `a` and `b` should not be const, so those are ignored.
let {a, b} = obj;
a = a + 1;

ignoreReadBeforeAssign

这是避免与no-use-before-define规则冲突的选项（无"nofunc"选项）。如果true指定，则此规则将忽略在声明和第一个赋值之间读取的变量。默认是false。

{"ignoreReadBeforeAssign": true}选项的正确代码示例：

/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": true}]*/
/*eslint-env es6*/

let timer;
function initialize() {
    if (foo()) {
        clearInterval(timer);
    }
}
timer = setInterval(initialize, 100);

默认{"ignoreReadBeforeAssign": false}选项的正确代码示例：

/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": false}]*/
/*eslint-env es6*/

const timer = setInterval(initialize, 100);
function initialize() {
    if (foo()) {
        clearInterval(timer);
    }
}

何时不使用它

如果您不想收到有关初始分配后永远不会重新分配的变量的通知，则可以安全地禁用此规则。

相关规则

• no-var

• no-use-before-define

版本

该规则在 ESLint 0.23.0中引入。

资源

• 规则资源

• 文档资源