let  a = [];
a[1] = 1;
console.log(a); // [empty, 1]

  function createEmptyMatrix(n) {
    const matrix = [];
    for (let i=0; i<n; i++) {
      matrix.push([]);
    }
    return matrix;
  }
  createEmptyMatrix(2) //[ [], [] ]

Runtime Complexity

Space Complexity

Fabonacci

function fib(n, arr = [0, 1]) {
if (arr.length >= n+1) return arr[n];
arr[arr.length] = arr[arr.length-1] + arr[arr.length-2];
return fib(n, arr);
}

// terrible horrible runtime complexity (exponential)!!!
function fib(n) {
  if (n < 2) return n;
  return fib(n-1) + fib(n-2);
}