js去尾法保留n位小数

在使用前添加这段代码

Number.prototype.toFloor = function (decimal) {
    let num = this.toString();
    let index = num.indexOf('.');
    if(index !== -1){
        num = num.substring(0, decimal + index + 1)
    } else {
        num = num.substring(0)
    }
    return parseFloat(num).toFixed(decimal)
}

String.prototype.toFloor = function (decimal) {
  let numberString = this
  if (isNaN( Number(numberString) )) {
    return '0'
  }
  if (-1===numberString.indexOf('.') || isNaN(parseInt(decimal)) ) {
    return numberString
  }
  const strings = numberString.split('.')
  if (parseInt(decimal)<1) {
    return strings[0]
  }
  return strings[0]+'.'+strings[1].substring(0, decimal)
}