One . Code

 

  function fn(num) {
    num = String(num)
    let str = ''
    for (let i = num.length - 1, j = 1; i >= 0; i--, j++) {
      if (j % 3 == 0 && i != 0) {
        str = str + num[i] + ','
        continue
      } else {
        str = str + num[i]
      }
    }
    return str.split('').reverse().join('')
  }
  let a1 = fn(1498498415)
  console.log(a1);

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Analysis of ideas :

It can be seen that the incoming number is first converted into a string .

i It's a decreasing form that leads to traversal

j Is an incremental form , Used to add... In place , Segmentation .

The steps of circular execution ：

5

5+1

5+1+4,

5+1+4,+8

5+1+4,+8+9,

5+1+4,+8+9+4,

......

So follow this step .

then :

 return str.split('').reverse().join('')

Cut strings into arrays , Flip , Convert to string .

Two . If not  continue What will happen?

continue, Jump out of current loop , Execute next cycle .

Code :

<script>
  function fn(num) {
    num = String(num)
    let str = ''
    for (let i = num.length - 1, j = 1; i >= 0; i--, j++) {
      if (j % 3 == 0 && i != 0) {
        str = str + num[i] + ','
        // continue
      } else {
        str = str + num[i]
      }
    }
    return str.split('').reverse().join('')
  }
  let a1 = fn(1234567890)
  console.log(a1);
</script>

It won't have any impact .

3、 ... and .i It's not equal to 0

stay j=3 When , here , Also to the third place , And split it up .

The answer is to add limiting conditions .

Because the format of the final result is :

  That is, the first one does not need to be segmented . therefore i Can not be equal to 0.

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