Declare a delegate type in Typescript

Question

Coming from a C# background, I want to create a datatype that defines a function signature. In C#, this is a delegate declared like this:

delegate void Greeter (string message);  public class Foo {     public void SayHi (Greeter g) {         g("Hi!");     } } 

Now, I want to achieve similar in Typescript. I know Typescript has no delegate types, but only lambdas. I came up with something like this:

class Foo {     SayHi (greeter: (msg: String) => void) {         greeter('Hi!');     } } 

While this works, I want to reuse the method signature (msg:String) => void couple of times and think it would be cleaner to create a custom type - like the delegate in C#.

Any ideas how this can be done?

Answer 1

In TypeScript, interfaces can have call signatures. In your example, you could declare it like this:

interface Greeter {     (message: string): void; }  function sayHi(greeter: Greeter) {     greeter('Hello!'); }  sayHi((msg) => console.log(msg)); // msg is inferred as string 

Answer 2

You can create something like a delegate by using a type alias:

type MyDelegate = (input: string) => void;

which defines a type name for a function pointer, just as delegates do in C#. The following example uses it in combination with generic type parameters:

type Predicate<T> = (item: T) => boolean;  export class List<T> extends Array<T> {     constructor(...items: T[]){         super();         for(let i of items || []){             this.push(i);         }     }     public hasAny(predicate?: Predicate<T>): boolean {         predicate = predicate || (i => true)         for(let item of this) {             if(predicate(item)) return true;         }         return false;     } }