A QUnit assert to verify that an object or expression changes correctly over time (i.e., browser animation frames).

watch-assert.js

// watch assert
//     expression: object reference or function
//     values:     an array of values
//     message:    QUnit assert message
    
//     each value in values is compared against expression for
//     a frame using requestAnimationFrame
    
QUnit.extend(QUnit.assert, {
    watch: function(expression, values, message) {
        var numSteps = values.length;
        var steps = 0;
        var val   = expression instanceof Function
            ? function() { return expression(); }
            : function() { return expression; }
        ;

        step();

        function step() {
            var a = val();
            var b = values[steps];
            QUnit.push(QUnit.equiv(a, b), a, b, message + ', step ' + steps);
            ++steps < numSteps
                ? requestAnimationFrame(step)
                : start()
            ;
        };
    }
});

QUnit.asserts.watch(function() { return itemList.children().length; }, [ 300, 600, 900, 1000, 1000 ],
'rendering long lists takes multiple frames, but eventually finishes');