思路就是通過(guò)str.slice(beginIndex[, endIndex])函數(shù)每3位截?cái)鄐tring處理加上,.
直接處理string效率上是高于轉(zhuǎn)為數(shù)組再轉(zhuǎn)回的.

const formatToThousands = num => {
    let number = num.toString();
    let result = '';
    while (number.length > 3) {
        result = `,${number.slice(-3)}${result}`; //從尾部開(kāi)始3位截?cái)嗵幚砑?,'
        number = number.slice(0, number.length -3); //number 截?cái)嗟粑膊?位
    }
    return `${number}${result}`
}

下面是處理可能包含小數(shù)的數(shù)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: 以上沒(méi)判斷傳入?yún)?shù)是否是數(shù)字, 以及負(fù)數(shù)情況.