1. 向下取整

parseInt();//直接去除小数部分

Math.floor();

2. 向上取整：

Math.ceil();//向下取整+1

3. 四舍五入

Math.round();

实例：

var number1 = number2 = number3 = number4 = number5 = 2.6;

console.log("5个原始的数据都为",number1);//2.6

console.log('正数部分')

console.log('直接去除小数向下取整',parseInt(number1));//2

console.log('向下取整',Math.floor(number2));//2

console.log('向上取整',Math.ceil(number3));//3

console.log('四舍五入',Math.round(number4));//3

console.log('负数部分')

console.log('直接去除小数向下取整',parseInt(-number1));//-2

console.log('向下取整',Math.floor(-number2));//-3

console.log('向上取整',Math.ceil(-number3));//-2

console.log('四舍五入',Math.round(-number4));//-3