prefer-const
要求const声明从未重新赋值的变量
🔧 可修复
此规则报告的一些问题可以通过--fix 命令行选项自动修复
如果变量从未重新赋值，则使用const声明更好。
const声明告诉读者，“此变量从未重新赋值”，从而减少认知负担并提高可维护性。
规则详细信息
此规则旨在标记使用let关键字声明但从未在初始赋值后重新赋值的变量。
此规则的错误代码示例
在游乐场中打开
/*eslint prefer-const: "error"*/
// it's initialized and never reassigned.
let = 3;
console.log(a);
let b;
= 0;
console.log(b);
class C {
static {
let a;
= 0;
console.log(a);
}
}
// `i` is redefined (not reassigned) on each loop step.
for (let in [1, 2, 3]) {
console.log(i);
}
// `a` is redefined (not reassigned) on each loop step.
for (let of [1, 2, 3]) {
console.log(a);
}
此规则的正确代码示例
在游乐场中打开
/*eslint prefer-const: "error"*/
// using const.
const a = 0;
// it's never initialized.
let b;
console.log(b);
// it's reassigned after initialized.
let c;
c = 0;
c = 1;
console.log(c);
// it's initialized in a different block from the declaration.
let d;
if (true) {
d = 0;
}
console.log(d);
// it's initialized in a different scope.
let e;
class C {
#x;
static {
e = obj => obj.#x;
}
}
// it's initialized at a place that we cannot write a variable declaration.
let f;
if (true) f = 0;
console.log(f);
// `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(i);
}
// `predicate` is only assigned once but cannot be separately declared as `const`
let predicate;
[object.type, predicate] = foo();
// `g` is only assigned once but cannot be separately declared as `const`
let g;
const h = {};
({ g, c: h.c } = func());
// suggest to use `no-var` rule.
var i = 3;
console.log(i);
选项
{
"prefer-const": ["error", {
"destructuring": "any",
"ignoreReadBeforeAssign": false
}]
}
解构
处理解构中变量的方式。有两个值
"any"（默认值） - 如果解构中的任何变量都应为const，则此规则会警告这些变量。"all"- 如果解构中的所有变量都应为const，则此规则会警告变量。否则，忽略它们。
默认{"destructuring": "any"}选项的错误代码示例
在游乐场中打开
/*eslint prefer-const: "error"*/
let {a, } = 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
在游乐场中打开
/*eslint prefer-const: "error"*/
// using const.
const {a: a0, b} = obj;
const a = a0 + 1;
// all variables are reassigned.
let {c, d} = obj;
c = c + 1;
d = d + 1;
{"destructuring": "all"}选项的错误代码示例
在游乐场中打开
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
// all of `a` and `b` should be const, so those are warned.
let {, } = obj; /*error 'a' is never reassigned, use 'const' instead.
'b' is never reassigned, use 'const' instead.*/
{"destructuring": "all"}选项的正确代码示例
在游乐场中打开
/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
// '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
这是一个选项，用于避免与no-use-before-define规则（无"nofunc"选项）冲突。如果指定了true，则此规则将忽略在声明和第一次赋值之间读取的变量。默认为false。
{"ignoreReadBeforeAssign": true}选项的正确代码示例
在游乐场中打开
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": true}]*/
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
在游乐场中打开
/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": false}]*/
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在初始赋值后从未重新赋值的变量的通知，可以安全地禁用此规则。
相关规则
版本
此规则在 ESLint v0.23.0 中引入。