JS实现千位计数格式

思路就是通过str.slice(beginIndex[, endIndex])函数每3位截断string处理加上,.
直接处理string效率上是高于转为数组再转回的.

const formatToThousands = num => {
    let number = num.toString();
    let result = '';
    while (number.length > 3) {
        result = `,${number.slice(-3)}${result}`; //从尾部开始3位截断处理加','
        number = number.slice(0, number.length -3); //number 截断掉尾部3位
    }
    return `${number}${result}`
}

下面是处理可能包含小数的数字

const formatToThousands = num => {
  let number = num.toString();
  let result = '';
  const doshPosition = number.indexOf(".");
  let integer = '', decimal = '';

  if(doshPosition !== -1) {
    integer = number.slice(0, doshPosition);
    decimal = number.slice(doshPosition);
  } else {
    integer = number;
  }

  while (integer.length > 3) {
    result = `,${integer.slice(-3)}${result}`;
    integer = integer.slice(0, integer.length -3);
  }

  return `${integer}${result}${decimal}`
}

tips: 以上没判断传入参数是否是数字, 以及负数情况.