Nested loops

A loop inside another runs completely for every round of the outer one: two loops of ten give a hundred rounds. That is how you fill a grid or visit every pair. Build a grid as an array of rows: make an empty row in the outer loop, fill it in the inner loop, push it to the result.

To pair items without repeats, start the inner index one past the outer index, for (let j = i + 1; ...).

for (let row = 0; row < 2; row++) {
  for (let col = 0; col < 3; col++) {
    console.log(row, col);
  }
}
// 0 0, 0 1, 0 2, 1 0, 1 1, 1 2

Your task

Write multiplicationTable(n) returning n rows where row i holds i * 1 to i * n, so multiplicationTable(3) is [[1, 2, 3], [2, 4, 6], [3, 6, 9]]. Then pairs(items) returning every pair once, in order, as two-element arrays: pairs(["a", "b", "c"]) is [["a", "b"], ["a", "c"], ["b", "c"]].

function multiplicationTable(n) {
  const rows = [];
  for (let i = 1; i <= n; i++) {
    const row = [];
    for (let j = 1; j <= n; j++) {
      row.push(i * j);
    }
    rows.push(row);
  }
  return rows;
}

function pairs(items) {
  const result = [];
  for (let i = 0; i < items.length; i++) {
    for (let j = i + 1; j < items.length; j++) {
      result.push([items[i], items[j]]);
    }
  }
  return result;
}