blowup.js

/**
 * Blowup
 * Write a function which takes a string and returns the blowup of said string.
 *
 * Definition. Blowup of a string.
 * The blowup of a string
 *  s = "a1a2a3..."
 * is defined by
 *  blowup = "a1a2a2a3a3a3..."
 * Flor example, the blowup of "bang!" is "baannngggg!!!!!"
 *
 * Test Cases
 *
 * > blowup "bang!"
 * "baannngggg!!!!!"
 *
 * > blowup "hi!"
 * "hii!!!"
 *
 * > blowup ""
 * ""
 */

function blowup(string) {
  return string.split("").reduce((blow, char, times) => {
    return blow + char.repeat(times + 1);
  }, "");
}