Function.prototype.bind()

bind方法用於將函數體內的this綁定到某個對象，而後返回一個新函數

var d = new Date();
d.getTime() // 1481869925657

var print = d.getTime;
print() // Uncaught TypeError: this is not a Date object.
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比較好的用處

var counter = {
        count: 0,
        inc: function () {
            this.count++;
        }
    };

    var obj = {
        count: 100
    };
    var func = counter.inc.bind(obj);
    func();
    obj.count // 101
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