JS Practice — Built-in Objects

Array[Symbol.species] — subclass concat()

📖 Usage Example

class MyArr extends Array {}
const a = new MyArr(1, 2)
const b = a.concat([3, 4])
b instanceof MyArr  // → true

concat() also respects Symbol.species when called on an Array subclass instance.

Array[Symbol.species] and concat()

The .concat() method also honours Symbol.species. When called on an Array subclass (and no species override is present), it returns an instance of the subclass.

Challenge: Create ConcatArray that extends Array. Implement concatReturnsSubclass that returns true when .concat() on a ConcatArray instance produces another ConcatArray instance.

concatReturnsSubclass() // → true

💡 Hints (2)

• `.concat()` creates a new array via `Symbol.species`, just like `.map()` and `.filter()`.
• No override is needed — subclasses inherit this behaviour automatically.