Locker Problem

locker.js

// fill an 100-element array with zero (0 = closed, 1 = open)
let lockers = new Array(100).fill(0)

// iterate 1-100 in outer loop, iterate i-100, jumping to every i-th term
for (let i = 1; i <= 100; i++) { 
  for (let j = i; j <= 100; j += i) { 
    // flip switch on locker value
    lockers[j - 1] = !lockers[j - 1] ? 1 : 0
  }
}

// output where arr[i] is truthy (1) then appends i + 1 to final array
console.log(lockers.reduce((p, c, i, arr) => { 
  if (arr[i]) { return p.concat(i + 1) }
  else { return p }
},  []))

// [ 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 ]