How do I extend object type inside function param with typescript?

Question

Let's say I have a type

type Template = {
    a: string;
    b: string;
    c: string
}

and I want to use it inside a function, but I want to add extra param to it. What would be the best way to implement it?

when I try to extend it, typescript says ? expected

here's how I do it

const test = (template: Template extends { d: string }[]) => {
    template.map(t => console.log(t.d));
}

P.S. I don't to uses [key:string]: string for this type

Answer 1

It's not possible to extend types. Ref this accepted answer

You can fix your problem something like this

type Template = {
    a: string;
    b: string;
    c: string
}

type NewTemplate = Template & { d: string }

const test = (template: NewTemplate[]) => {
    template.map(t => console.log(t.d));
}

Answer 2

If you don't want to use '&' you extend the type, you can make the function generic:

const test = <T extends Template>(template: T[]) => {
    template.map(t => console.log(t.d))
}