Check if an array is a subset of another array

Return true when every value of subset appears in of.

JavaScript Version 1.0.0 · MIT · .md

function isSubset({ subset, of }) { const all = new Set(of); return subset.every(value => all.has(value)); }

Input & expected output

{
  "subset": [
    "[email protected]"
  ],
  "of": [
    "[email protected]",
    "[email protected]"
  ]
}

true

The edge case

{
  "subset": [
    "[email protected]"
  ],
  "of": []
}

false

Know the limits

• The original comment said “b is a subset of a” while its code tested the opposite, and its second variant compared joined strings. Named fields remove the ambiguity.
• Values compare with SameValueZero; objects by reference.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Check whether all values are equal — Compare every item with the first using Object.is.
• Check whether an object is empty — Check for own enumerable string keys on a JSON object.
• Check whether an integer is even — Test parity for safe integers, including zero and negative values.