Question: I know that String and Number objects are immutable but are **all** built-in objects immutable as well?
If it's not true, can you give me an example of a built-in object that is mutable?

Answer: 
Arrays are mutable. Here are a list of mutable methods for arrays.

`push` - Elements are added on to the end of the array

`pop` - Elements are removed from the end of the array

`shift` - Elements are removed from the beginning of the array

`unshift` - Elements are added to the beginning of the array

`splice` - Adds/removes elements to an array

None of these methods create a new array, but modify the existing array.
Nope, for instance an array is mutable. If you have an array of strings and you push a string to it, you don't get a new array with an extra string added to it. The original array is altered.