no-useless-assignment
不使用值时禁止变量赋值
维基百科描述了 “死存储” 如下：
¥Wikipedia describes a “dead store” as follows:
在计算机编程中，已分配值但不被任何后续指令读取的局部变量称为死存储。
¥In computer programming, a local variable that is assigned a value but is not read by any subsequent instruction is referred to as a dead store.
“死存储” 浪费处理和内存，因此最好删除对变量不必要的赋值。
¥"Dead stores" waste processing and memory, so it is better to remove unnecessary assignments to variables.
另外，如果作者打算使用该变量，则死存储周围可能存在错误。例如，
¥Also, if the author intended the variable to be used, there is likely a mistake around the dead store. For example,
-
你应该使用存储值但忘记这样做。
¥you should have used a stored value but forgot to do so.
-
你在要存储的变量名称中犯了错误。
¥you made a mistake in the name of the variable to be stored.
let id = "x1234"; // this is a "dead store" - this value ("x1234") is never read
id = generateId();
doSomethingWith(id);
规则详情
¥Rule Details
此规则旨在在未使用值时报告变量分配。
¥This rule aims to report variable assignments when the value is not used.
此规则的错误代码示例：
¥Examples of incorrect code for this rule:
/* eslint no-useless-assignment: "error" */
function fn1() {
let v = 'used';
doSomething(v);
v = 'unused';
}
function fn2() {
let v = 'used';
if (condition) {
v = 'unused';
return
}
doSomething(v);
}
function fn3() {
let v = 'used';
if (condition) {
doSomething(v);
} else {
v = 'unused';
}
}
function fn4() {
let v = 'unused';
if (condition) {
v = 'used';
doSomething(v);
return
}
}
function fn5() {
let v = 'used';
if (condition) {
let v = 'used';
console.log(v);
v = 'unused';
}
console.log(v);
}
此规则的正确代码示例：
¥Examples of correct code for this rule:
/* eslint no-useless-assignment: "error" */
function fn1() {
let v = 'used';
doSomething(v);
v = 'used-2';
doSomething(v);
}
function fn2() {
let v = 'used';
if (condition) {
v = 'used-2';
doSomething(v);
return
}
doSomething(v);
}
function fn3() {
let v = 'used';
if (condition) {
doSomething(v);
} else {
v = 'used-2';
doSomething(v);
}
}
function fn4() {
let v = 'used';
for (let i = 0; i < 10; i++) {
doSomething(v);
v = 'used in next iteration';
}
}
该规则不会报告从未读取过的变量。因为它显然是一个未使用的变量。如果你希望报告，请启用 no-unused-vars 规则。
¥This rule will not report variables that are never read. Because it’s clearly an unused variable. If you want it reported, please enable the no-unused-vars rule.
/* eslint no-useless-assignment: "error" */
function fn() {
let v = 'unused';
v = 'unused-2'
doSomething();
}
何时不使用
¥When Not To Use It
如果你不想收到有关从未读取的值的通知，你可以安全地禁用此规则。
¥If you don’t want to be notified about values that are never read, you can safely disable this rule.
相关规则
版本
此规则是在 ESLint v9.0.0-alpha.1 中引入。