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) or let otherVariable = a;
• it's called or constructed, like a() or new 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";