js 一元操作符

p36:
The unary plus and minus are familiar symbols to most people and operate the same way in
ECMAScript as they do in high school math. The unary plus essentially has no effect on a number:
var iNum= 25;
iNum = +iNum;
alert(iNum); //outputs “25”
In this code, the unary plus is applied to the number 25, which returns the exact same value. Although
unary plus has no effect on numbers, it has an interesting effect on strings: It converts them to numbers.
var sNum = “25”;
alert(typeof sNum); //outputs “string”
var iNum = +sNum;
alert(typeof iNum); //outputs “number”

这在jquery的src中有看到

function now(){
    return +new Date;
}