When would you use a Map or Set instead of a plain object or array?

Use a Map when keys are not strings, or when you add and remove keys often. Use a Set when you need unique values or fast "is it in here?" checks.

Map:

• Keys can be any value, including objects and functions. Plain object keys are always strings or symbols, so obj[{}] turns the key into '[object Object]'.
• Map compares keys with SameValueZero. This is like ===, except NaN equals NaN.
• It keeps insertion order.
• It has a size property.
• You can loop over it directly.
• It has no inherited keys like constructor or __proto__ that could collide with your keys.
• It is optimized for frequent additions and deletions.

Set:

• It stores unique values.
• It is the standard way to remove duplicates: [...new Set(arr)].
• Its has lookup is fast. array.includes is linear, which means it checks items one by one.

Plain objects are still better for fixed-shape records, like a user with id and name. They are also better for anything you need to serialize: JSON.stringify turns a Map or Set into {}.