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Here you go, another JavaScript approach, originally posted on SO:

function getRepString (rep) {

  rep = rep+''; // coerce to string

  if (rep < 1000) { // return the same number
    return rep; 
  }

  if (rep < 10000) { // place a comma between
  
    return rep.charAt(0) + ',' + rep.substring(1);
  } 

  // divide and format
  return (rep/1000).toFixed(rep % 1000 != 0)+'k';

}

Check the output results here.