Question

I have the following problem with .push() method:

var myArray = ["a", "b", "c", "d"];

function add(arr) {
    arr.push("e");
    return arr;
}

add(myArray);

// myArray is now  ["a", "b", "c", "d", "e"]

Why it overrides myArray? Can’t understand that…

Answer 1

Arrays in Javascript (and most other languages) are passed by reference.

When you write add(myArray), you are passing a reference to the same Array instance that the global myArray variable refers to.
Any changes to the array object will be visible through both references.

To copy the actual array instance, write add(myArray.slice());.
Note that this will not copy the objects inside of it.