PermCheck Javascript 100%/100%

permcheck.js

function solution(A) {
  var L = A.length; // Length of the array
  var X = ((L + 1) * L) / 2; // Sum from 1..L
  var Y = 0; // Sum of evaluated values
  var I = 0; // Counter
  var F = []; // Found elements
  var V = -1; // Container
    
  while (I < L) { // Start searching the array
    V = A[I]; // Get the value
    I++; // Increase the counter
    if (F[V]) continue; // If we've accounted for this, continue
    F[V] = true; // Record the value
    Y += V; // Add to the evaluated sum
  }
    
  // If the evaluated sum is equal to the sum of the array length
  // then we can be sure that the array is a permutation.
  return X === Y ? 1 : 0;
}