A unit testable version of debounce from underscore.js I think underscore needs a flush feature so we can easily test our applications that use it. A underscore.flush method would allow you to test the different cases without having to make your unit tests slow. This could work similar to angular.js's $httpBackend.flush().

underscore-test.js

describe('underscore', function(){

  it('shouldnt execute immediately', function(){
    var hasHappened = false;

    var fn = underscore.debounce(function(){
        hasHappened = true;
    }, 100);

    expect(hasHappened).toBe(false);

    fn();

    expect(hasHappened).toBe(false);
  });

  it('should execute after delay', function(){
    var hasHappened = false;
    runs(function(){
        var fn = underscore.debounce(function(){
            hasHappened = true;
        }, 100);

        fn();
    });

    waitsFor(function(){
        return hasHappened;
    }, 'didnt execute', 200);

    runs(function(){
        expect(hasHappened).toBe(true);
    });
  });

  it('should only happen once', function(){
    var count = 0;
    runs(function(){
        var fn = underscore.debounce(function(){
            count += 1;
        }, 100);

        fn();
        fn();
        fn();
    });

    waitsFor(function(){
        return count > 0;
    }, 'didnt execute', 200);

    runs(function(){
        expect(count).toBe(1);
    });
  });

  it('can flush', function(){
    var hasHappened = false;
    var fn = underscore.debounce(function(){
        hasHappened = true;
    }, 100);

    fn();
    underscore.flush();

    expect(hasHappened).toBe(true);
  });

  it('can debounce independent functions', function(){
    var aHappened = false;
    var bHappened = false;

    var a = underscore.debounce(function(){
        aHappened = true;
    }, 100);

    var b = underscore.debounce(function(){
        bHappened = true;
    }, 100);

    a();
    b();

    underscore.flush();

    expect(aHappened).toBe(true);
    expect(bHappened).toBe(true);
  });
});

underscore.js

var underscore = {};
var timeouts = {};

underscore.debounce = function(fn, delay){
  var timeoutId;

	return function(){
		delete timeouts[timeoutId];
		clearTimeout(timeoutId);

		timeoutId = setTimeout(function(){
			fn();
		}, delay);

		timeouts[timeoutId] = fn;
	};
};

underscore.flush = function(){
	for(var timeoutId in timeouts){
		timeouts[timeoutId]();
		delete timeouts[timeoutId];
		clearTimeout(timeoutId);
	}
};

Thanks. It was very useful for me