arrays - Number formatting in JavaScript -

i trying create js script format numbers using , separator going bad in logic implementation.

i changing values of arr1 here.

this js code using time -

<script> var arr1 = [1,2,3,4,5,6,7]; arr1.reverse(); var = 1; var temparr = new array(); for( i; <= arr1.length ; i++ ) {     if( i%3 == 0 ) {         temparr[i-1] = arr1[i-1];         temparr[i] = ',';         i++;     }     else {         temparr[i-1] = arr1[i-1];     } } console.log(temparr.reverse().join('')); </script> 

expected output `` current wrong output

1,234           =>  ,234  12,345          =>  1,345 123,456         =>  ,12,456 1,234,567       =>  ,23,567 

kindly let me know doing wrong(logical part) in snippet learning coding time.

you've forgotten add original number comma:

var arr1 = [1,2,3,4,5,6,7].reverse()   , temparr = []; (var = 0; ++i <= arr1.length;) {     temparr[i-1] = arr1[i-1];     if (i % 3 === 0) temparr[i] = arr1[i++] + ','; } console.log(temparr.reverse().join('')); 

a more concise version:

function addcommas(str) {     str = (str + '').split('');     (var = str.length - 1; (i -= 3) > 0;) str[i] += ',';     return str.join(''); }