defaultParameterLast

Enforce default parameters to be last.

✅ This rule is included in the ts untyped preset.

Putting default parameters last allows function calls to omit optional tail arguments. When default parameters appear before required ones, callers must explicitly pass undefined to skip them.

This rule reports parameters with default values or optional markers (?) that appear before required parameters.

Examples

❌ Incorrect

function greet(name = "World", greeting: string) {
  console.log(`${greeting}, ${name}!`);
}

function calculate(a = 1, b: number, c = 2) {
  return a + b + c;
}

const fn = (x = 0, y: number) => x + y;

function example(a?: number, b: number) {
  return (a ?? 0) + b;
}

✅ Correct

function greet(greeting: string, name = "World") {
  console.log(`${greeting}, ${name}!`);
}

function calculate(b: number, a = 1, c = 2) {
  return a + b + c;
}

const fn = (y: number, x = 0) => x + y;

function example(a = 1, ...rest: number[]) {
  return rest.reduce((sum, value) => sum + value, a);
}

Options

This rule is not configurable.

When Not To Use It

If your API design intentionally places default parameters before required ones for readability or documentation purposes, you may disable this rule. Some legacy APIs may also have established parameter orderings that cannot be changed without breaking changes.

Further Reading

• MDN documentation on Default parameters

Equivalents in Other Linters

• Biome: useDefaultParameterLast
• Deno: default-param-last
• ESLint: default-param-last@typescript-eslint/default-param-last
• Oxlint: eslint/default-param-last