JavaScript array.every() method returns false when using the in operator?

Question

Below is the isSubSet() function that checks if elements from subset B and subset C are in subset A are in the universal set.

const isSubSet = (universalSet,subSet) => (
  subSet.every(element => element in universalSet)
)
const [setA,setB,setC ] = [ [1,2,3,4,5], [1,2,3],[1,2,3,4,5]]
const [checkA, checkB ] = [,isSubSet(setA, setC)]

console.log(`is setB subset of setA: ${isSubSet(setA, setB)}`)
console.log(`is setC subset of setA: ${isSubSet(setA, setC)}`)

Does the array.every() return true for proper subsets only?

score 3 · Accepted Answer · answered Apr 06 '20 at 12:57

You need to check with Array#includes.

The in operator checks if a key of an object exists.

const
    isSubSet = (universalSet, subSet) =>
        subSet.every(element => universalSet.includes(element));

const [setA, setB, setC] = [[1, 2, 3, 4, 5], [1, 2, 3], [1, 2, 3, 4, 5]]
const [checkA, checkB] = [, isSubSet(setA, setC)]

console.log(`is setB subset of setA: ${isSubSet(setA, setB)}`)
console.log(`is setC subset of setA: ${isSubSet(setA, setC)}`)

JavaScript array.every() method returns false when using the in operator?

1 Answers1