isContainedIn javascript Code Snippet in 2025

Checks if the elements of the first array are contained in the second one regardless of order.

• Use a for...of loop over a Set created from the first array.
• Use Array.prototype.some() to check if all distinct values are contained in the second array.
• Use Array.prototype.filter() to compare the number of occurrences of each distinct value in both arrays.
• Return false if the count of any element is greater in the first array than the second one, true otherwise.

const isContainedIn = (a, b) => {
  for (const v of new Set(a)) {
    if (
      !b.some(e => e === v) ||
      a.filter(e => e === v).length > b.filter(e => e === v).length
    )
      return false;
  }
  return true;
};

isContainedIn([1, 4], [2, 4, 1]); // true