AI solution for JavaScript Coding Question on BFE.dev
65. add comma to number

Solution: We can first convert the number to a string and then insert commas after every three digits from the right using a regular expression.

Here's the code:

function addComma(num) {
  // convert number to string
  const str = num.toString();
  // use regex to add commas
  return str.replace(/(\d)(?=(\d{3})+(?!\d))/g, '$1,');
}

Explanation:

• We first convert the number to a string using toString().
• Then we use the replace() method of the string to search for groups of three digits from the right using a regular expression. The regular expression matches any digit (\d) that is followed by one or more groups of three digits (\d{3}) and is not followed by any more digits ((?!\d)). The g flag at the end tells the replace() method to replace all occurrences, not just the first one.
• Inside the replace() method, the replacement string '$1,' adds a comma after each matched digit. Here, $1 represents the first group of the matched string, which is a single digit.

Finally, we return the modified string containing commas as thousand separators.