no-useless-assignment
Disallow variable assignments when the value is not used
Wikipedia describes a “dead store” as follows:
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.
“Dead stores” waste processing and memory, so it is better to remove unnecessary assignments to variables.
Also, if the author intended the variable to be used, there is likely a mistake around the dead store. For example,
- you should have used a stored value but forgot to do so.
- 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);
Rule Details
This rule aims to report variable assignments when the value is not used.
Examples of incorrect code for this rule:
Open in Playground
/* eslint no-useless-assignment: "error" */
function fn1() {
let v = 'used';
doSomething(v);
= 'unused';
}
function fn2() {
let v = 'used';
if (condition) {
= 'unused';
return
}
doSomething(v);
}
function fn3() {
let v = 'used';
if (condition) {
doSomething(v);
} else {
= 'unused';
}
}
function fn4() {
let = 'unused';
if (condition) {
v = 'used';
doSomething(v);
return
}
}
function fn5() {
let v = 'used';
if (condition) {
let v = 'used';
console.log(v);
= 'unused';
}
console.log(v);
}
Examples of correct code for this rule:
Open in Playground
/* 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';
}
}
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.
Open in Playground
/* eslint no-useless-assignment: "error" */
function fn() {
let v = 'unused';
v = 'unused-2'
doSomething();
}
When Not To Use It
If you don’t want to be notified about values that are never read, you can safely disable this rule.
Related Rules
Version
This rule was introduced in ESLint v9.0.0-alpha.1.
Further Reading