Mastering TypeScript 3 - Third Edition

Spying on callback functions

Let's see how we can use a spy to test whether a callback function was invoked correctly. Consider the following TypeScript code:

class CallbackClass { 
    doCallback(id: number, callback: (result: string) => void ) { 
        let callbackValue = "id:" + id.toString(); 
        callback(callbackValue); 
    } 
} 
 
class DoCallback { 
    logValue(value: string) { 
        console.log(value); 
    } 
} 

Firstly, we define a class named CallbackClass that has a single function, doCallback. This doCallback function takes an id argument, of type number, and also a callback function. The callback function must take a string as an argument and return void.

The second class that we have defined, named DoCallBack, has a single function, named logValue. This function ...