JS 数字类型的加减乘除, 四舍五入保持精度

Number.prototype.toFixed = function (d = 0) {
	let changeNum
        = this + ''// 把数字转为字符串
	if (changeNum.indexOf('-') != -1){// 判断是否是负数
	  changeNum = Math.abs(Number(changeNum))
  }
	changeNum = (Math.round(Number(changeNum) * Math.pow(10, d)) / Math.pow(10, d)).toString()
	const index = changeNum.indexOf('.')// 判断字符是否有小数点
	if (index === -1) { // 如果没有小数点，加上对应个数的小数点
		changeNum += '.'
		changeNum += new Array(d + 1).join('0')
	} else if (index > 0) { // 如果有，index是小数点的坐标值，比如在第二位index=1
		const temp = d - (changeNum.length - index - 1)// 小数点后的位数小于要取舍的个数，用0补齐；数字的长度-小数点的坐标值-1=小数的位数(小数点也占以为，4.00 length:4;.的index是1)
		if (temp > 0) {
			changeNum += new Array(temp + 1).join('0')
		}
	}
	if (this.toString().indexOf('-') != -1) { // 如果是负数，加上-
		changeNum = '-' + changeNum
	}
	return changeNum
}
Number.prototype.add = function (arg2){
  const arg1 = this 
	let r1, r2
	try {
		r1 = arg1.toString().split('.')[1].length
	} catch (e) {
		r1 = 0
	}
	try {
		r2 = arg2.toString().split('.')[1].length
	} catch (e) {
		r2 = 0
	}
	const m = Math.pow(10, Math.max(r1, r2))

	return (arg1 * m + arg2 * m) / m
}
Number.prototype.sub = function (arg2){
  const arg1 = this
	let r1, r2
	try {
		r1 = arg1.toString().split('.')[1].length
	} catch (e) {
		r1 = 0
	}
	try {
		r2 = arg2.toString().split('.')[1].length
	} catch (e) {
		r2 = 0
	}
	const m = Math.pow(10, Math.max(r1, r2)) // 动态控制精度长度
	const n = (r1 >= r2) ? r1 : r2
	return parseFloat(((arg1 * m - arg2 * m) / m).toFixed(n))
}
Number.prototype.mul = function(arg2){
  const arg1 = this
	let m = 0
	const s1 = arg1.toString()
	const s2 = arg2.toString()
  try{
    m += s1.split('.')[1].length
  }catch(e){
    m +=0
  }
  try{
    m += s2.split('.')[1].length
  }catch(e){
    m +=0
  }
	return Number(s1.replace('.', '')) * Number(s2.replace('.', '')) / Math.pow(10, m)
}
Number.prototype.div = function(arg2){
  const arg1 = this
	let t1 = 0
	let t2 = 0
  try{
    t1 = this.toString().split('.')[1].length
  }catch(e){
    t1 = 0
  }
  try{
    t2 = arg2.toString().split('.')[1].length
  }catch(e){
    t2 = 0
  }
	const r1 = Number(arg1.toString().replace('.', ''))
	const r2 = Number(arg2.toString().replace('.', ''))
	return (r1 / r2) * Math.pow(10, t2 - t1)
}
Number.prototype.toLocalString = function(num = 0){
  let str = this.toLocaleString()
  if(num > 0){
    str = str.split('.')[0] + '.' + this.toFixed(num).split('.')[1]
  }
  return str
}

// 0.41
(0.19).add(0.22)