Using parameters and data type in Typescript

Question

Using parameters and data type in Typescript

When I remove

<IFirst extends {}, ISecond extends {}>

from the declaration of this function, the compiler generates an error. Isn't the return value supposed to be the type after the double dot? What does

<IFirst extends {}, ISecond extends {}>

mean after the function name? Why do I need to include both

<IFirst extends {}, ISecond extends {}>

and : IFirst & ISecond in the declaration? I've searched through the documentation and online resources, but I haven't been able to find a satisfactory answer.

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

typescript function

Answer 1

Answer №1

& represents an intersection type in TypeScript, categorized as one of the advanced types.

This type combines all properties found in both IFirst and ISecond interfaces.

In a generic type like

<IFirst extends {}, ISecond extends {}>

, the keyword extends indicates that the parameters must match an empty object {}.

This function takes in two parameters that can be converted to an empty object {}, ensuring that the return value will include properties from both input objects.

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

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

console.log(merged);

The variable merged will have a type definition of:

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

Answer 2

& represents an intersection type in TypeScript, categorized as one of the advanced types.

This type combines all properties found in both IFirst and ISecond interfaces.

In a generic type like

<IFirst extends {}, ISecond extends {}>

, the keyword extends indicates that the parameters must match an empty object {}.

This function takes in two parameters that can be converted to an empty object {}, ensuring that the return value will include properties from both input objects.

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

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

console.log(merged);

The variable merged will have a type definition of:

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

Using parameters and data type in Typescript

Answer №1

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