Checking if a string belongs to an array type value to a key in an object

Say I have an object called "groups", in it there are keys whose values are arrays:

let groups = {
    group1: ['A', 'B', 'C'],
    group2: ['X', 'Y', 'Z']
};
let item = 'B';

How would I go about finding out that the item is in fact in group1? (Note: There would be a lot of groups. I'm aware I could use a for loop to iterate over each of the keys in the object. But would there be a faster way?)

via Wright