JS里面的一元加号操作符

1.转换为时间戳

<p>
        <label>getTime:</label><input id="getTime" type="text" readonly value=""/>
    </p>
    <p>
        <label>valueOf:</label><input id="valueOf" type="text" readonly value=""/>
    </p>
    <p>
        <label>unaryPlus +:</label><input id="unaryPlus" type="text" readonly value=""/>
    </p>
    <p>
        <label>now:</label><input id="now" type="text" readonly value=""/>
    </p>

        var now = new Date(),
            now_time = Date.now();
        var getTime=document.getElementById("getTime"),
            unaryPlus=document.getElementById("unaryPlus"),
            valueOf=document.getElementById("valueOf"),
            onow=document.getElementById("now");
        onow.value= now_time;
        getTime.value=now.getTime();
        unaryPlus.value= +now;
        valueOf.value= now.valueOf();

        console.log(now.getTime() === now.valueOf())//true
        console.log(now.getTime() === +now)//true
        console.log(now.getTime() === now_time)//true

对于用+转换和原生转换性能查看：http://jsperf.com/gettime-vs-unaryplus

2.强制为数字

+true //1

+null //0

+"" //0

+"3.0" //3

+"4.2" //4.2

特殊：

+"3asdf"  //NAN

parseInt("02a2sdf", 10)  //2

举例：

20 + "15" //2015

20 + +"15" //35