unicorn/no-useless-undefined

A variable without a value, a return without a value and a missing argument are already undefined, and a parameter defaults to undefined anyway. Writing it out changes nothing.

--fix removes the undefined.

Configure

Off by default. To turn it on:

[lint.rules]
"unicorn/no-useless-undefined" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"unicorn/no-useless-undefined" = ["error", {
  checkArguments = true,
  checkArrowFunctionBody = true,
}]

Examples

Reported:

let selected = undefined;

function find(id, fallback = undefined) {
  if (!id) {
    return undefined;
  }
  return load(id, undefined);
}

const noop = () => undefined;

Not reported:

let selected;

function find(id, fallback) {
  if (!id) {
    return;
  }
  return load(id);
}

const noop = () => {};

Sometimes you do need to write it. A const must have a value. A function with a return type keeps its return undefined, because TypeScript may require it. And you can only drop undefined at the end of the arguments.

const missing = undefined;

function find(id: string): string | undefined {
  return undefined;
}

render(undefined, container);

Calls where the argument is the point aren't reported either. That covers functions whose name starts with set, collection methods such as push, add, has and includes, assertions such as toBe and toEqual, createContext, and ref.

setUser(undefined);
items.push(undefined);
expect(result).toBe(undefined);

Options

Option	Default	What it does
checkArguments	true	Report undefined at the end of a call's arguments.
checkArrowFunctionBody	true	Report () => undefined.

load(id, undefined);

const noop = () => undefined;