Shorter code isn't always better code.

bigArr.js

const bigArr = new Array(1000000).fill(1).map((_, i) => i);
bigArr[100] = 5;

const allUnique = arr => {
  const all = new Set();

  for (let el of arr) {
    if (all.has(el)) {
      return false;
    }

    all.add(el);
  }

  return true;
}

const allUnique2 = arr => arr.length === [...new Set(arr)].length;

console.time("first");
allUnique(bigArr);
console.timeEnd("first");

console.time("second");
allUnique2(bigArr);
console.timeEnd("second");

// VM969:22 first: 0.109130859375ms
// VM969:26 second: 138.579833984375ms