#no-useless-assignment

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-useless-assignment': 'error',
    },
  },
]);

#Rule Details

Disallows assignments whose value is never read. A variable that is written and then overwritten — or written on a path that ends without reading it again — carries a value nothing observes, which usually means a mistake: a typo in the variable name, a missing return, or leftover code from a refactor.

The rule only looks at variables it can follow end to end. It stays silent when the variable is never read at all (that is no-unused-vars' job), when it is read from another function, when it is exported, and when the assignment sits inside a try block, where the block may be abandoned before the value is used.

Examples of incorrect code for this rule:

function fn() {
	let v = 'used';
	console.log(v);
	v = 'unused';
}

function fn() {
	let v = 'unused';
	if (condition) {
		v = 'used';
		console.log(v);
		return;
	}
}

function fn() {
	let v = 'used';
	console.log(v);
	v = 'unused';
	v = 'used';
	console.log(v);
}

Examples of correct code for this rule:

function fn() {
	let v = 'used';
	console.log(v);
	v = 'used-2';
	console.log(v);
}

function fn() {
	let v = 'used';
	if (condition) {
		v = 'used-2';
		console.log(v);
		return;
	}
	console.log(v);
}

function fn() {
	let v = 'used';
	console.log(v);
	setTimeout(() => console.log(v), 1);
	v = 'used in another scope';
}

#Differences from ESLint

• The /* exported foo */ directive comment is not supported. In a script file, ESLint treats a variable named by it as observable from outside and reports no assignments to it; this rule still analyzes the variable.
• Destructuring follows the run-time evaluation order: the initializer or right-hand side produces its value first, then each element evaluates its computed key and default. So let { a = x } = (x = 1, obj) is clean here — the default may read x = 1 — while ESLint reports it based on source positions; and in ({ [a = 1]: x } = (a = 2, obj)); console.log(a) this rule reports the genuinely dead a = 2 where ESLint reports the a = 1 the log actually observes.
• Every label wrapped around a loop resolves its break/continue: outer: inner: while (cond) { … continue outer; } keeps its back edge, so values the next iteration reads are counted as used. ESLint attaches only the innermost label and falsely reports such assignments.
• A variable read from inside a parameter decorator's arguments is treated as used, so assignments to it are never reported: const pipe = {}; class C { handler(@Body(pipe) body) {} }. ESLint reports const pipe = {} here — a known false positive (eslint/eslint#20947).
• For deeply nested try/catch/switch combinations, this rule's control flow modeling can occasionally diverge from ESLint's own, which has open, accepted-but-unfixed bugs in the same area (eslint/eslint#17579) and its own false negatives on some such shapes. Matching it exactly there is hard to pin down, so it is left as a known gap. These shapes are rare in practice; in the large majority of real code this rule reports the same findings as ESLint.

#Original Documentation

• ESLint no-useless-assignment