Function parameters and type in Typescript

Question

In the declaration of this function, if I remove <IFirst extends {}, ISecond extends {}>, the compiler reports an error; is not the return value the type after the double dot? What's the meaning of <IFirst extends {}, ISecond extends {}> after the name of the function? Why I have to put both <IFirst extends {}, ISecond extends {}> and : IFirst & ISecond on the declaration? I checked the documentation and surfed the Internet, but I cannot find the answer.

function extend<IFirst extends {}, ISecond extends {}>(
    IFirst: IFirst,
    ISecond: ISecond
): IFirst & ISecond {}

@Evaldas Buinauskas: sorry; question updated; format problems. — user1
– user1, Commented Aug 19, 2019 at 14:31

Evaldas Buinauskas · Accepted Answer · 2019-08-19 14:58:22Z

1

& is a intersection type and it's one of the advanced types in TypeScript.

It's going to combine all properties from IFirst and ISecond into one.

<IFirst extends {}, ISecond extends {}> is a generic type and extends keyword means that IFirst or ISecond has to be compatible with {}.

So basically this function accepts two parameters that can be cast to {} and will ensure that return type will have properties from both first and second objects.

function extend<TFirst extends {}, TSecond extends {}>(
    first: TFirst,
    second: TSecond
): TFirst & TSecond {
  return Object.assign({}, first, second);
}

const extended = extend({a: 1, b: 2 }, { c: 3, d: 4 });

console.log(extended);

extended will have a definition of:

const extended: {
    a: number;
    b: number;
} & {
    c: number;
    d: number;
}

answered Aug 19, 2019 at 14:58

Evaldas Buinauskas

14.2k11 gold badges63 silver badges119 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user1 Over a year ago

so <TFirst extends {}, TSecond extends {}> refers to the input parameters and not the output parameters, right?

Evaldas Buinauskas Over a year ago

In your case it dictates both, because it dictates the input and the output (intersection type). But normally you can think of them as dictating input parameters.

KBell Over a year ago

@Yellow75 You should read TypeScript documentation for more information about generics ;) And more specifically

Collectives™ on Stack Overflow

Function parameters and type in Typescript

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related