//ECMAScript 6 feature
let chars = new Set(["A", "B", "C", "D"]);

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}`);