TS2393

Duplicate function implementation

Duplicate function implementation.

The compiler’s own words. Not translated — this is the string you pasted into a search box.

Two function bodies share one name in the same scope; overloads may have many signatures but only one implementation.

Reproduction

function parse(value: string) {
  return value
}

function parse(value: number) {
  return value
}

The build asserts this emits exactly this code.

Why the compiler says this

JavaScript does not dispatch between same-named function declarations by parameter type; the later declaration would replace the earlier one. TypeScript overloads therefore separate several type-only signatures from one emitted body.

Fixes

function parse(value: string): string
function parse(value: number): number
function parse(value: string | number): string | number {
  return value
}

Write overload signatures followed by one implementation broad enough to handle them all.

function parseText(value: string) {
  return value
}

function parseNumber(value: number) {
  return value
}

Give different operations different names when they do not need one overloaded API.

Takeaway

An overload set has many declarations but exactly one body.

Where to go next

Generic parameters Try it in the Lab

Errors