fake-progress库的源码如下：

var FakeProgress = function (opts) {
    if (!opts) { opts = {}; }
    this.timeConstant = opts.timeConstant || 1000;
    this.autoStart = opts.autoStart || false;
    this.parent = opts.parent;
    this.parentStart = opts.parentStart;
    this.parentEnd = opts.parentEnd;
    this.progress = 0;
    this._intervalFrequency = 100;
    this._running = false;
    if (this.autoStart) {
        this.start();
    }
}

FakeProgress.prototype.start = function () {
    this._time = 0;
    this._intervalId = setInterval(this._onInterval.bind(this), this._intervalFrequency);
};

FakeProgress.prototype._onInterval = function () {
    this._time += this._intervalFrequency;
    this.setProgress(1 - Math.exp((-1 * this._time) / this.timeConstant));
};

FakeProgress.prototype.end = function () {
    this.stop();
    this.setProgress(1);
};

FakeProgress.prototype.stop = function () {
    clearInterval(this._intervalId);
    this._intervalId = null;
};

FakeProgress.prototype.createSubProgress = function (opts) {
    var parentStart = opts.start || this.progress;
    var parentEnd = opts.end || 1;
    var options = Object.assign({}, opts, {
        parent: this,
        parentStart: parentStart,
        parentEnd: parentEnd,
        start: null,
        end: null
    });
    var subProgress = new FakeProgress(options);
    return subProgress;
};

FakeProgress.prototype.setProgress = function (progress) {
    this.progress = progress;
    if (this.parent) { this.parent.setProgress((this.parentEnd - this.parentStart) * this.progress + this.parentStart); }
};

我们需要核心关注的参数只有timeConstant，autoStart这两个参数，通过阅读源码可以知道timeConstant相当于分母，分母越大则加的越少，而autoStart则是一个开关，如果开启了直接执行start方法，开启累计的定时器。通过这个库，我们实现一个虚拟的进度条，永远到达不了100%的进度条。

调用如下：

 

 

var fake = new FakeProgress({
        timeConstant: 1000,
        autoStart: true
    });

Object.defineProperties(fake, { progress: {
        get: function () {
            return progress;
        },
        set: function (val) {
            progress = val;
            var _proNum = parseInt(progress * 100);
            if (_proNum >= 99) {
                clearInterval(this._intervalId);
                this._intervalId = null;
            }
            $('#_progressBarBulletbox').progressbar('setValue', _proNum);
        }
    }
    });

function pageLoadBarDownFile(progressBarId) {
        progressBarId = progressBarId || "_progressBarBulletbox";
        var _width = $(window).width() / 2;
        var html = "<div id=\"" + progressBarId + "\" class=\"easyui-progressbar\" data-options=\"value:0\" style=\"width:" + _width + "px;\"></div>";
        $("<div class=\"datagrid-mask\"></div>").css({ display: "block", width: "100%", height: $(window).height() }).appendTo("body");
        $(html).appendTo("body").css({ display: "block", height: '10%', width: '50%', position: 'fixed', top: 0, right: 0, bottom: 0, left: 0, margin: 'auto', left: 0 });
    }

如果需要实现一个永远不满的进度条，那么你可以借助fake-progress核心是1 - Math.exp((-1 * this._time) / this.timeConstant) 这个公式
涉及到一个数据公式： e的负无穷次方 趋近于0。所以1-e^-x永远到不了1，但趋近于1

核心原理就是：用时间做分子，传入的timeConstant做分母，通过Math.exp((-1 * this._time) / this.timeConstant) 可知，如果时间不断累积且为负值，那么Math.exp((-1 * this._time) / this.timeConstant) 就无限趋近于0。所以1 - Math.exp((-1 * this._time) / this.timeConstant) 就可以得到无限趋近于1 的值。