Declare a delegate type in Typescript

c# types lambda delegates typescript
Dynalon picture Dynalon · Dec 1, 2013 · Viewed 35.2k times · Source

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

Ryan Cavanaugh picture Ryan Cavanaugh · Dec 1, 2013

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

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