typescript arguments of function code example

Example 1: typescript default parameter

// Default Parameters
sayHello(hello: string = 'hello') { 
    console.log(hello); 
}

sayHello(); // Prints 'hello'

sayHello('world'); // Prints 'world'

Example 2: typescript parameter function type

class Foo {
    save(callback: (n: number) => any) : void {
        callback(42);
    }
}
var foo = new Foo();

var strCallback = (result: string) : void => {
    alert(result);
}
var numCallback = (result: number) : void => {
    alert(result.toString());
}

foo.save(strCallback); // not OK
foo.save(numCallback); // OK

Example 3: typescript set argument type

// non typed function
function add(x, y) {
  return x + y;
}

// typed function
function add(x: number, y: number): number {
  return x + y;
}

// types can be found here: https://www.typescriptlang.org/docs/handbook/basic-types.html