JS implementation thousand separators

// Regular Expressions implement thousands separators function format (num) {    var reg =/\d{1,3} (? = (\d{3}) +$)/g;     return (num + "). Replace (Reg, ' $&, ');} Console.log (Format (13123903243));  // 13,123,903,243

Explain:

The regular Expression \d{1,3} (? = (\d{3}) +$) represents a number preceded by at least one set of 3 digits.

? = indicates a forward reference, which can be used as a matching condition, but the matched content is not fetched and is the start of the next query.

The $& represents the content that matches the regular expression.

//General loop method implements thousand separatorsfunctionformat1 (num) {num= num + ';//Numeric to String    varstr = "";  for(varI=num.length-1,j=1; i>=0; I--, J + +){        if(J%3==0 && i!=0) {//every three bits plus a comma, filter exactly the first number of casesstr + = Num[i] + ', ';//add thousand decimal points comma            Continue; } STR+=Num[i]; }    returnStr.split ("). Reverse (). Join (');//String = = Array = Invert = String}console.log (FORMAT1 (13123903243));//13,123,903,243

JS implementation thousand separators