const counterIncrement = (collector, char) => {
  const lastItemIndex = collector.length - 1;

  if(lastItemIndex === -1) {
    return [[char, 1]];
  }

  if(collector[lastItemIndex][0] == char) {
    collector[lastItemIndex][1] += 1
  } else {
    collector.push([char, 1]);
  }

  return collector;
}

const charCounters = (input) => input.split('').reduce(counterIncrement, []);

const output = (charCounters) => (
  charCounters.map(([char, counter]) => `${char}${counter}`).join(' ')
);

const assert = require('assert');

assert(output(charCounters('aaaaabbvva')) == "a5 b2 v2 a1");

assert(
  output(charCounters('aaaaaaaaaqqqqqqqfewfv')) == "a9 q7 f1 e1 w1 f1 v1"
);