@typescript-eslint/no-useless-default-assignment

A default value only applies when the value is undefined. If the type says that a property, a tuple element, or a callback parameter is always there, the default never runs, and it tells readers that the value can be missing when it can't.

A default of undefined is reported too, because a missing value is already undefined.

Configure#

Off by default. This rule needs types, so turning it on makes bun lint type check your code first.

[lint.rules]
"@typescript-eslint/no-useless-default-assignment" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"@typescript-eslint/no-useless-default-assignment" = ["error", {
  allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing = false,
}]

Examples#

function greet({ name = "guest" }: { name: string }) {
  return `Hello, ${name}`;
}

const doubled = [1, 2, 3].map((value = 0) => value * 2);

const point: [number, number] = [1, 2];
const [x = 0] = point;

function connect(port = undefined) {}

const { host = undefined } = { host: "localhost" };

function greet({ name = "guest" }: { name?: string }) {
  return `Hello, ${name}`;
}

const doubled = [1, 2, 3].map(value => value * 2);

In a function declaration, the default is what makes the parameter optional, so it's allowed:

function connect(port: number = 3000) {}

connect();

Use ? to make a parameter optional, not = undefined:

function connect(port: number | undefined = undefined) {}

function connect(port?: number) {}

Options#

Without strictNullChecks in tsconfig.json, types don't say what can be undefined. By default, the rule reports each of those files once, to tell you to turn it on:

const port = 3000;