Wrong function type when mixing default initialized parameters with regular parameters

[luisabreu]

VS Code does not show the correct parameter types on the tooltip when you mix default initialized parameters with regular parameters. Ex.:

function test(name= "John", age: number){}

The tooltip shown when you put the mouse cursor over test says (name: string, age: number): void instead of (name?:string, age: number).

[mhegazy]

Optional parameters can not appear before required ones. so nome it is not optional. it just has a default parameter.

[luisabreu]

in my example, name is not an optional parameter. It's a default initialized parameter. According to the docs:

"Unlike plain optional parameters, default-initialized parameters don’t need to occur after required parameters. If a default-initialized parameter comes before a required parameter, users need to explicitly pass undefined to get the default initialized value. "

https://www.typescriptlang.org/docs/handbook/functions.html

The problem is that the tooltip is showing the wrong information, though the control flow is being performed correctly.

[mhegazy]

Defaults do not show in signature anywhere else, they are considered function implementation detail.;so I am not sure I understand the request here.

[RyanCavanaugh]

@mhegazy I think when a default exists, we should be pushing | undefined into the parameter type

[mhegazy]

is --strictNullChecks on?

[mhegazy]

With --strictNullCheck yes it should be T | undefined.

[luisabreu]

Yep, strict null check is on.

[sandersn]

Fix is up at #12033

[akarzazi]

function AddProduct(product: string, quantity = 1) {
  // I think quantity should be number;
}

If default initialized parameter is automatically converted to T | undefined.
How can you enforce T as type ?

[blakeembrey]

@akarzazi Inside the function would still be T, while the call signature is T | undefined as it has a default initialiser (the default value is used when the argument is undefined).