Basic genetic algorithm in JS

<button onclick="start()">START</button>
<button onclick="stop()">STOP</button>
<button onclick="init()">INIT</button>

<p>
  <b><span id="target"></span></b>
  </br>
  <span id="best"></span>
  <ul>
    <li>
      mutation rate : <span id="mutationRate"></span>
    </li>
    <li>
      average fitness: <span id="avgFitness"></span>
    </li>
    <li>
      total population: <span id="maxPop"></span>
    </li>
    <li>
      total generations : <span id="counter"></span>
    </li>
  </ul>

</p>
<div id="genetic"></div>
<script>

// 1. initialize
const MUTATION_RATE = 0.01,
  SPEED = 100

document.getElementById("mutationRate").innerHTML = MUTATION_RATE*100 + "%"

var target = "how happy days",
  popMax = 150,
  timer,
  i,
  mutants = getInitialPopulation(target);

// show target
document.getElementById("target").innerHTML = target;

init(target)
start()

// population of random elements
function init(target) {
  mutants = getInitialPopulation(target);
  i = 0;
}

function start () {
  timer = setInterval(() => {
    draw()
  }, SPEED);
}

function stop() {
  clearInterval(timer)
}

function draw() {
  mutants = newGeneration(mutants)
  if (mutants.indexOf(target) !== -1) stop()
}

function getInitialPopulation(target) {
  return [...new Array(popMax)]
    .map( d =>
      Array(Math.floor(Math.random() * target.length)+1) // random length
      .fill("a")
      .map( d => getRandomLetter() )
      .join("")
    )
}

function newGeneration(population) {
  let matePool = []

let fitness = population.map(e => calcFitness(e, target) )
  let avgFitness = fitness.reduce((a, b) => a + b ) / fitness.length
  let best = getBest(fitness)

let maxFitness = Math.max.apply(null, fitness);

// 2. selection
  population
    .forEach( (e, i) =>  {
      // - evaluate fitness of each element
      let fit = map(fitness[i],0,maxFitness,0,0.1);
      let pool = Array(Math.floor(fit*100)).fill(e)
      matePool = [ ...matePool, ...pool]
    })

let mutants = population.map((a) => {
    // - pick two parents
    let i = Math.floor(Math.random()*matePool.length);
    let j = Math.floor(Math.random()*matePool.length);

// - crossover (create a child by combining DNA)
    let child = crossover(matePool[j],matePool[j])

// - mutation (mutate the child based on a given probability)
    let mutant = mutate(child)

// - add the new child to the population
    return mutant
  })

let bestMutant = getBest(mutants.map(e => calcFitness(e, target)) )
  show(mutants, i++, avgFitness, mutants[bestMutant])

return mutants
}

function getBest(fitness){
  return fitness.indexOf(Math.max(...fitness));
}

function calcFitness (source, target) {
  let score = 0;

source.split("").forEach( (letter, i) => 
    target[i] === letter ? score ++ : null
  )
  return score / target.split("").length;
}
console.assert(calcFitness("love", "love") === 1)
console.assert(calcFitness("1ove", "love") === .75)

function crossover(a,b) {
  let len = target.split("").length;
  let midpoint = Math.floor(Math.random()*len) // Pick a midpoint

// Half from one, half from the other
  let child  = a.split("").slice(0,midpoint).join("")
    + b.split("").slice(midpoint,len).join("")

console.log(child.length)
  return child
}

function map (num, in_min, in_max, out_min, out_max) {
  return (num - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

// Based on a mutation probability, picks a new random character
function mutate(child) {
 return child
   .split("")
   .map( letter =>
    Math.random() < MUTATION_RATE
    ?
    getRandomLetter()
    :
    letter
   )
   .join("")
}

function getRandomLetter() {
  return " abcdefghijklmnopqrstuvwxyz"[Math.floor(Math.random() * 26)]
}

function show(population, i, avgFitness, best) {
  var li = population.map(d => "<li>" + d + "</li>").join("")
  document.getElementById("genetic").innerHTML = "<ul style='columns: 4'>"+li+"</ul>"
  document.getElementById("counter").innerHTML = i + " generations"
  document.getElementById("avgFitness").innerHTML = Math.round(avgFitness*100)/100
  document.getElementById("best").innerHTML = best
  document.getElementById("maxPop").innerHTML = population.length
}
</script>