How to round js values

1.toFixed() method can round Number to a number with specified decimal places.

NumberObject.toFixed(num)

num must be written, specifying the number of decimal places, which is a value between 0 ~ 20, including 0 and 20. Some implementations can support a larger range of values. If this parameter is omitted, 0 will be used instead.

当num超过20的时候，js会出错，这东西好像只能传一个数字进去，字符串会爆不是一个方法

Bug in the method:

Number(13.35).toFixed(1); //13.3 
Number(0.055).toFixed(1); //0.1
原因：原生toFixed(x)截取小数的时候会有误差
//如果要修改这个缺陷，可以把js中的number类型的tofixed方法重写。

2.Math.round() Method rounds a number to the nearest integer.

Math.round(x)

x is the value to be calculated. This method returns the nearest integer to the given numeric expression.

Existing bug

##

Math.round(num * Math.pow(10, 2)) / Math.pow(10, 2);  //num是待处理数字Math.pow(10, 2)=100
当num = 10.10500时，计算上述表达式可得10.11。（正常）
当num = "10.50000"时（注意这里是字符串），计算上述表达式可得10.5。（damn it，小数点后位数不对！）

原因：当num是字符串，进行乘法操作时，进行了类型转换，后缀零被丢弃了，导致位数不足，这个时候我们就应该进行补0

Solution:

var iTofixed =function(num,fractionDigits) {
 return (Math.round(num*Math.pow(10,fractionDigits))/Math.pow(10,fractionDigits)+Math.pow(10,-(fractionDigits+1))).toString().slice(0, -1)
};
iTofixed('13.5000',3);
//"13.500"

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