How jasmine clock works?

Question

I don't want to read code for hours to find the relevant part, but I am curious how jasmine implements its clock. The interesting thing with it is that it can test async code with sync testing code. AFAIK, with the current node.js, which supports ES5, this is not possible (async functions are defined in ES7). Does it parse the js code with something like estraverse and build an async test from the sync one?

Just an example of what I am talking about:

it("can test async code with sync testing code", function () {
    jasmine.clock().install();

    var i = 0;
    var asyncIncrease = function () {
        setTimeout(function () {
            ++i;
        }, 1);
    };

    expect(i).toBe(0);
    asyncIncrease();
    expect(i).toBe(0);
    jasmine.clock().tick(2);
    expect(i).toBe(1);

    jasmine.clock().uninstall();
});

In here the expect(i).toBe(1); should be in a callback.

Answer 1

The install() function actually replaces setTimeout with a mock function that jasmine gives you more control over. This makes it synchronous, because no actual waiting is done. Instead, you manually move it forward with the tick() function, which is also synchronous.

See the source code: https://github.com/jasmine/jasmine/blob/ce9600a3f63f68fb75447eb10d62fe07da83d04d/src/core/Clock.js#L21

Suppose you had a function that internally set a timeout of 5 hours. Jasmine just replaces that setTimeout call so that the callback will be called when you call tick() so that the internal counter reaches or exceeds the 5 hour mark. It's quite simple!