Javascript skill evaluation

Correct answer

by Alexandre Azevedo

HTML

<script src="//searls.github.io/jasmine-all/jasmine-all-min.js"></script>
<script type="application/javascript">
	'use strict';

    describe('Declaration', function() {
		it('is function', function() {
        	var type = typeof(window.MyFunction);
            expect(type).toBe('function');
        });
    });

    describe('Execution of', function() {
    
    	var value;

        it("initial sequence", function() {
	        value = new window.MyFunction();
            expect(value.current()).toBe(1);
        });

		it("incrementing sequence once", function() {
	        value.increment();
            expect(value.current()).toBe(1);
        });

		it("incrementing sequence three times", function() {
	        value.increment().increment().increment().increment();
            expect(value.current()).toBe(8);
        });

		it("decrementing sequence two times", function() {
	        value.decrement().decrement();
            expect(value.current()).toBe(3);
        });
    });

</script>

JavaScript

'use strict';

window.MyFunction = function() {
    this._previous 	= 0;
    this._current  	= 1;
};
    
window.MyFunction.prototype.current = function() {
    return this._current;
};
    
window.MyFunction.prototype.increment = function() {
    this._current  += this._previous;
    this._previous  = this._current - this._previous;
    
    return this;
};
    
window.MyFunction.prototype.decrement = function() {
    this._previous  = this._current - this._previous;
    this._current  -= this._previous;
    
    return this;
};