TS1015

Question mark and default

Parameter cannot have question mark and initializer.

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

A default value already makes a parameter optional, so the question mark is not merely redundant — it contradicts the guarantee the default is there to give.

Reproduction

function greet(name?: string = 'world') {
  return name.toUpperCase()
}

The build asserts this emits exactly this code.

Why the compiler says this

The two marks make different promises about the same parameter. `?` says the argument may be left out, and that inside the body the parameter may therefore be `undefined`. `= "world"` says the argument may be left out and the parameter will be `"world"` instead — never `undefined`. The body can only be checked against one of those, so TypeScript refuses to record both and asks which one you meant.

Fixes

01
```
function greet(name: string = 'world') {
  return name.toUpperCase()
}
```
Keep the default and drop the question mark. The parameter is still optional at the call site; it is simply never `undefined` inside.
02
```
function greet(name?: string) {
  return (name ?? 'world').toUpperCase()
}
```
Or keep the question mark and handle the absence yourself. Choose this when the body needs to know whether the caller said anything at all.

Takeaway

A default is the stronger promise: optional at the call site, never `undefined` inside. The question mark only buys a case you then have to handle.

Where to go next

Modifiers Try it in the Lab

Errors