新手入门啊，看看就明白了！
<script src='jquery.js'></script>
......
<script>
var a=1;
var b='2';
//output number
console.log(typeof(-b));
console.log(typeof(+b));
//output string
console.log(typeof(b));
//output 12
console.log(a + b);
//output 3
console.log(a + Number(b));
console.log(a + +b);
console.log(a - -b);
//output -1
console.log(a + -b);
console.log(a - +b);
</script>