Can figure out the difference using spread operator in array and object in JavaScript

Question

I am using spread operator in an array like below:

const addOne = false;
const arr = [1, 2, 3]
const arr1 = [...arr, ...(addOne && [8, 9])];
console.log(arr1)

And I got the error

TypeError: (addOne && [8,9]) is not iterable
    at <anonymous>:4:32

While if I use spread operator in an object like:

const addOne = false;

const obj = {
  name: 'peter',
  age: 23
}
const obj1 = {
  ...obj,
  ...(addOne && obj)
}
console.log(obj1)

it prints out the correct output

Anyone knows why?

Answer 1

(false && obj) evaluates to a boolean so its' not iterable.
When using the spread operator when building your obj1 object, js uses a different method to extract the key value pairs from an object (remember that a bool value in js is still an object), so it won't throw an error since it's expecting an object as an input.