index.js

// Based on Easing Equations (c) 2003 Robert Penner, all rights reserved.
// This work is subject to the terms in
// http://www.robertpenner.com/easing_terms_of_use.html
// Preview: http://www.robertpenner.com/Easing/easing_demo.html
//
// Thanks to:
//  - https://github.com/yui/yui3/blob/master/src/anim/js/anim-easing.js
//  - https://github.com/gilmoreorless/jquery-easing-molecules


var PI = Math.PI;
var pow = Math.pow;
var sin = Math.sin;
var MAGIC_NUM = 1.70158; // Penner's magic number


/**
 * 和 YUI 的 Easing 相比，这里的 Easing 进行了归一化处理，参数调整为：
 * @param {Number} t Time value used to compute current value 0 =< t <= 1
 * @param {Number} b Starting value  b = 0
 * @param {Number} c Delta between start and end values  c = 1
 * @param {Number} d Total length of animation d = 1
 */
var Easing = {

    /**
     * Uniform speed between points.
     */
    easeNone: function(t) {
        return t;
    },

    /**
     * Begins slowly and accelerates towards end. (quadratic)
     */
    easeIn: function(t) {
        return t * t;
    },

    /**
     * Begins quickly and decelerates towards end.  (quadratic)
     */
    easeOut: function(t) {
        return (2 - t) * t;
    },

    /**
     * Begins slowly and decelerates towards end. (quadratic)
     */
    easeBoth: function(t) {
        return (t *= 2) < 1 ?
                .5 * t * t :
                .5 * (1 - (--t) * (t - 2));
    },

    /**
     * Begins slowly and accelerates towards end. (quartic)
     */
    easeInStrong: function(t) {
        return t * t * t * t;
    },
    /**
     * Begins quickly and decelerates towards end.  (quartic)
     */
    easeOutStrong: function(t) {
        return 1 - (--t) * t * t * t;
    },

    /**
     * Begins slowly and decelerates towards end. (quartic)
     */
    easeBothStrong: function(t) {
        return (t *= 2) < 1 ?
                .5 * t * t * t * t :
                .5 * (2 - (t -= 2) * t * t * t);
    },

    /**
     * Backtracks slightly, then reverses direction and moves to end.
     */
    backIn: function(t) {
        if (t === 1) t -= .001;
        return t * t * ((MAGIC_NUM + 1) * t - MAGIC_NUM);
    },

    /**
     * Overshoots end, then reverses and comes back to end.
     */
    backOut: function(t) {
        return (t -= 1) * t * ((MAGIC_NUM + 1) * t + MAGIC_NUM) + 1;
    },

    /**
     * Backtracks slightly, then reverses direction, overshoots end,
     * then reverses and comes back to end.
     */
    backBoth: function(t) {
        var s = MAGIC_NUM;
        var m = (s *= 1.525) + 1;

        if ((t *= 2 ) < 1) {
            return .5 * (t * t * (m * t - s));
        }
        return .5 * ((t -= 2) * t * (m * t + s) + 2);
    },

    /**
     * Snap in elastic effect.
     */
    elasticIn: function(t) {
        var p = .3, s = p / 4;
        if (t === 0 || t === 1) return t;
        return -(pow(2, 10 * (t -= 1)) * sin((t - s) * (2 * PI) / p));
    },

    /**
     * Snap out elastic effect.
     */
    elasticOut: function(t) {
        var p = .3, s = p / 4;
        if (t === 0 || t === 1) return t;
        return pow(2, -10 * t) * sin((t - s) * (2 * PI) / p) + 1;
    },

    /**
     * Snap both elastic effect.
     */
    elasticBoth: function(t) {
        var p = .45, s = p / 4;
        if (t === 0 || (t *= 2) === 2) return t;

        if (t < 1) {
            return -.5 * (pow(2, 10 * (t -= 1)) *
                    sin((t - s) * (2 * PI) / p));
        }
        return pow(2, -10 * (t -= 1)) *
                sin((t - s) * (2 * PI) / p) * .5 + 1;
    },

    /**
     * Bounce off of start.
     */
    bounceIn: function(t) {
        return 1 - Easing.bounceOut(1 - t);
    },

    /**
     * Bounces off end.
     */
    bounceOut: function(t) {
        var s = 7.5625, r;

        if (t < (1 / 2.75)) {
            r = s * t * t;
        }
        else if (t < (2 / 2.75)) {
            r = s * (t -= (1.5 / 2.75)) * t + .75;
        }
        else if (t < (2.5 / 2.75)) {
            r = s * (t -= (2.25 / 2.75)) * t + .9375;
        }
        else {
            r = s * (t -= (2.625 / 2.75)) * t + .984375;
        }

        return r;
    },

    /**
     * Bounces off start and end.
     */
    bounceBoth: function(t) {
        if (t < .5) {
            return Easing.bounceIn(t * 2) * .5;
        }
        return Easing.bounceOut(t * 2 - 1) * .5 + .5;
    }
};

// 可以通过 require 获取
module.exports = Easing;


// 也可以直接通过 jQuery.easing 来使用
var $ = require('jquery');
$.extend($.easing, Easing);