no-unused-vars
Recommended
Enforces all variables are used at least once.
Enforces all variables are used at least once.
If there are variables that are declared but not used anywhere, it's most likely because of incomplete refactoring. This lint rule detects and warns such unused variables.
Variable a
is considered to be "used" if any of the following conditions are
satisfied:
- its value is read out, like
console.log(a)
orlet otherVariable = a;
- it's called or constructed, like
a()
ornew a()
- it's exported, like
export const a = 42;
If a variable is just assigned to a value but never read out, then it's considered to be "not used".
let a;
a = 42;
// `a` is never read out
If you want to declare unused variables intentionally, prefix them with the
underscore character _
, like _a
. This rule ignores variables that are
prefixed with _
.
Invalid:
const a = 0;
const b = 0; // this `b` is never used
function foo() {
const b = 1; // this `b` is used
console.log(b);
}
foo();
let c = 2;
c = 3;
// recursive function calls are not considered to be used, because only when `d`
// is called from outside the function body can we say that `d` is actually
// called after all.
function d() {
d();
}
// `x` is never used
export function e(x: number): number {
return 42;
}
const f = "unused variable";
Valid:
const a = 0;
console.log(a);
const b = 0;
function foo() {
const b = 1;
console.log(b);
}
foo();
console.log(b);
let c = 2;
c = 3;
console.log(c);
function d() {
d();
}
d();
export function e(x: number): number {
return x + 42;
}
export const f = "exported variable";