CODEWITHSUNDEEP
javascripttypescript
manas
manas

Reputation: 6400

A function returning multiple types comma separated . What is it in Typescript?

I am new to Typescript I came across the following syntax

interface Foo {

    // what is age is here and 
    // what if it is not optional i.e () => string, age:number;

    (): () => string, age?: number;
}

What I have understood Foo is a function interface, any function that implements Foo must return a function that must return a string.

but what age?: number is for.

Upvotes: 2

Views: 434

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074495

It's clearer to understand with a line break, and a semicolon instead of comma (I'm intrigued that the comma is valid, but according to the playground, it is):

interface Foo {
    (): () => string;
    age?: number;
}

Foo defines an interface for a function that:

  • Returns a function that returns a string, and

  • Has an optional age property that's a number.

More about function types here.

Here's an example of it in use:

interface Foo {
    (): () => string,
    age?: number;
}

const f: Foo = () => () => "foo";
f.age = 42; // Note this is on the function

Playground link

Fair to say that's an...interesting...interface. :-)

Upvotes: 4

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