JS Bin// source http://jsbin.com/turutux

lookSay.js

"use strict";

function lookSay (number) {
  /* Setup:
   * Stringify the number, split it,
   * then coerce each string to an integer 
   */
  let split = `${number}`.split("").map(n => +n);
  let acc   = {};
  // '.reduce()' would be nice but it's not as efficient
  for (let i = 0; i < split.length; i++) {
    // if the current index is the same as the next index,
    if (split[i] === split[i + 1]) {
      // and if the accumulator doesn't have a corresponding property,
      if (!acc[split[i]]) {
        // initialize that property, and set it to 1.
        acc[split[i]] = 1;
      }
      else {
        // otherwise, increment that property.
        acc[split[i]]++;
      }
    }
    else {
      /* if the current index is NOT the same as the next index,
       * then we know that we're about to start indexing a new integer,
       * and should increment the accumulator's corresponding property 
       */
      acc[split[i]]++;
    }
  }
  // Build the output string 
  let output  = "";
  for (let j in acc) {
    output = output.concat(`${acc[j]}`, `${j}`);
  }
  /* Return parsed output.
   * Gain a small performance boost using the unary `+` operator:
   * https://jsperf.com/number-vs-parseint-vs-plus/19
   * (kinda negligible since we only call the operator twice in this function)
   */
  return +output;
}
// console.log(lookSay(3333111222222));