JS里面的一元加号操作符

<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