Function to determine if intersecting keys are equal

areIntersectingKeysEqual.js

export function areIntersectingKeysEqual(...objects) {
  return objects
    .map((object) => Object.keys(object))
    .sort((a, b) => a.length - b.length)
    .reduce((a, b) => a.filter((key) => b.includes(key)))
    .every((key) => objects.every((object) => object[key] === objects[0][key]));
}

Let me explain how it works.

At a high level, first we find the intersecting keys, then we determine if all intersecting keys are equal.

1. Map all objects, returning an array of key arrays.
2. Sort the lists according to length ascending (this is an optimization, shortest key list is filtered first)
3. Reduce our lists of keys, filtering them by the condition that the next array includes that key. This is how we find intersecting keys.
4. Now that we have a list of intersecting keys, we can determine if the keys are equal. For every key, check that the value in every object is equal.
5. Finally, return the resulting boolean.