js字符串转数值，向上取整，向下取整，四舍五入等方法

1.字符串转数值 Number()

var a = '3.14'
console.log(Number(a)) //3.14

2.只保留整数部分 parseInt()

var a = 3.14
console.log(parseInt(a)) //3

3.向下取整 Math.floor()

var a = 3.14
console.log(Math.floor(a)) //3

4.向上取整 Math.ceil()

var a = 3.14
console.log(Math.ceil(a)) //4

5.四舍五入 Math.round()

var a = 3.14
var b = 3.6
console.log(Math.round(a),Math.round(b)) //3,4