let chars = new Set(["A", "B", "C"])

let containA = chars.has('A')
//containA is true

let containE = chars.has('E')
//containE is false

let chars2 = new Set(["A", "B"])
let containAll = [...chars2]
    .filter(x => !chars.has(x))
    .length === 0
//containAll is true

console.log(`containA is ${containA}`)
console.log(`containE is ${containE}`)
console.log(`containAll is ${containAll}`)