CODEWITHSUNDEEP
javascriptjasmine
batman
batman

Reputation: 3685

How to inspect the javascript functions and its arguments

I'm very new to JS and I have been playing around with Jasmine.

In Jasmine, I can see a method called spyOn, which does inspect/spy the functions.

How does this works in js? Coming from Java background is it a proxy? How to write one?

Upvotes: 1

Views: 255

Answers (1)

balpha
balpha

Reputation: 50958

You can find the precise implementation on GitHub, but here is a simplified explanation:

function mySpy(obj, methodName) {
    // remember the original method
    var originalMethod = obj[methodName];

    // ... then replace it with a method that ...
    obj[methodName] = function () {

        // ... does whatever additional thing it wants to do ...
        console.log(methodName + " called, first argument: " + arguments[0]);

        // ... and then calls the original method with the same arguments,
        // and returns the result.
        return originalMethod.apply(this, arguments);
    };
}

Now you can do this:

var o = {
    inc: function (x) { return x + 1; }
};
mySpy(o, "inc");
console.log(o.inc(13));

This will output

inc called, first argument: 13
14

Three important things for you to know, coming from a Java background, are

  1. In JavaScript, it is not a problem to change an object's methods after the fact, dynamically. Calling someObj.someMethod = someOtherFunction is perfectly valid. (To be 100% precise, you may not actually be overwriting the original method, because it may be somewhere up the prototype chain, instead of on the object itself. That's an advanced topic though, and not very important here. Also, Java's distinction beween methods and class members doesn't apply to JavaScript.)
  2. The special "variable" arguments inside a function contains whatever arguments the function was called with. In Java terms, imagine that someMethod(Foo x1, Bar x2) always has an implicit second signature of the type someMethod(Object... arguments), meaning you could always use x1 and arguments[0] interchangeably.
  3. obj.someName and obj["someName"] are entirely equivalent in JavaScript. Because of this, you can easily access/change an object's properties using the property name as a string, something that in Java you would have to use reflection for.

Upvotes: 2

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