JS Flying Animation, Accelerated Animation, Swap In and Out Effects

<!DOCTYPE html>
<body>
 <div id="gift-fly" style=" position: absolute; width: 50px;height: 50px;background-color: #327dff "> Animation Box</div>
</body>
<script>
    let flyDiv = document.getElementById("gift-fly")
    animate(flyDiv, {"left": 500}, 3000, "easeIn").then(res => {
        console.log("Flying in animation completed")
        animate(flyDiv, {"left": 0}, 3000, "easeOut").then(res => {
            console.log("Flying out animation completed")
        })
    })

    /**
     * @param elem {HTMLElement} The HTML element to animate
     * @param params {JSON} HTML attributes to modify during the animation
     * @param duration {Number} Optional, duration of the animation in milliseconds
     * @param easing {String} Optional, easing mode of the animation, easing in easeIn, easing out easeOut
     */
    function animate(elem, params, duration, easing) {
        return new Promise(success => {
            /*
             * Description: tween animation algorithm.
             * @param Number t: The time the animation has been running (actually how many times/frames)
             * @param Number b: Starting position
             * @param Number c: Ending position
             * @param Number d: Time it takes from starting position to ending position (actually how many times/frames)
             */
            let tween = {
                easeIn: function (t, b, c, d) {// easing in
                    return c * (t /= d) * t + b;
                },
                easeOut: function (t, b, c, d) {// easing out
                    return -c * (t /= d) * (t - 2) + b;
                }
            };
            let attribute = {
                get: function (elem, attr) {
                    let val;
                    if (elem.currentStyle) {
                        if (attr === "opacity") {
                            val = elem.filters.alpha[attr];
                        } else {
                            val = elem.currentStyle[attr];
                        }
                    } else {
                        val = getComputedStyle(elem)[attr];
                        if (attr === "opacity") {
                            val = 100 * val;
                        }
                    }
                    return val;
                },
                set: function (elem, attr, val) {
                    if (attr == 'opacity') {
                        elem.style.filter = 'alpha(opacity=' + (val) + ')';
                        elem.style.opacity = (val) / 100;
                    } else {
                        elem.style[attr] = val + 'px';
                    }
                }
            };
            let effect = {
                animate: function (elem, params, duration, easing) {
                    let dt = new Date().getTime(),
                        b = 0,
                        c = 0,
                        d = duration || 500,
                        fps = 1000 / 60;

                    let changes = [];

                    for (let attr in params) {
                        b = parseFloat(attribute.get(elem, attr));
                        c = params[attr] - b;

                        changes.push({

                            attr: attr,

                            b: b,

                            c: c
                        });
                    }

                    easing = easing || "easeOut";
                    setTimeout(function () {
                        let t = new Date().getTime() - dt;
                        let b, c, attr;
                        for (let i = 0; i < changes.length; i++) {
                            b = changes[i].b;
                            c = changes[i].c;
                            attr = changes[i].attr;

                            attribute.set(elem, attr, tween[easing](t, b, c, d));
                            if (d <= t) {
                                attribute.set(elem, attr, params[attr]);
                                success();
                                return;
                            }
                        }
                        setTimeout(arguments.callee, fps);
                    }, fps);
                }
            };
            effect.animate(elem, params, duration, easing);
        })
    }
</script>
</html>