Partition arrays in Javascript

My latest browser game is done, I have deployed it to a temporary website and will share the link to it soon. Anyway, while building it, I came across this little problem where I was trying to partition arrays. As usual, there’s more than one way to solve the problem. In fact, there are 3 ways to solve it. Let’s start with the least efficient solution.

Partition arrays

Inefficient i.e. n squared runtime. See we have to go (iterate) through all elements of the array twice.

Inefficient approach (using filter)

let correctAns = questionStack.filter(val => {
  return val.userAns === val.correctAnswer;
});
let wrongAns = questionStack.filter(val => {
  return val.userAns !== val.correctAnswer;
});

Old Fashioned (using old for loop)

Old fashioned but runs in linear time and I kinda like it.

let correctAns;
let wrongAns;
for(let i=0; i < questionStack.length; i++) {
  let curQ = questionStack[i];
  if(curQ.userAns == curQ.correctAnswer) {
    correctAns.push(curQ);
  } else {
    wrongAns.push(curQ);
  }
}

Modern (using reduce)

The one below is the by far the shortest and also the least readable in my opinion.

Array.prototype.partitionWith = function(f) {
  return this.reduce((a, x) => a[f(x)+0].push(x) && a, [[],[]])
}

var isRight = x => x.userAns === x.correctAnswer;
var [right, wrong] = questionStack.partitionWith(isRight);
console.log(right);
console.log(wrong);

However it does run in linear time i.e. partitions the array in just one pass. The unreadability aspect is just my personal opinion a lot of Javascript developers would love this approach. To me, it’s less readable and harder to debug.

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