tojsexponent.js

var pattern = /^(\d+?)(0+)$/;

function ne(n) {
  'use strict';
  
  var str = String(n);
  var match = str.match(pattern);
  var len;

  if (!match) {
    return str;
  }

  len = match[2].length;

  if (len > 3) {
    if (match[1] === '1') {
      match[1] += '0';
      len -= 1;
    }
    str = match[1] + 'e' + len;
  }

  return str;
};

console.log(ne(10e1)); // "100"
console.log(ne(10e3)); // "1000"
console.log(ne(10e4)); // "10e4" etc.