working with copies of arrays in javascript

Question

When do variables made from arrays act as pointers to the array and when do they act as copies of the array?

For instance if I have an array named Array1

a1=Array1; 

is a1 a copy or a pointer to the array.

If I modify a1 will it also modify Array1. By modify I mean change a value, push something into the array, sort, or any other way you could modify an array.

thanks,

• Marc B: x = [1,2,3]; y = x; y[0] = 7; console.log(x) will output 7,2,3. arrays in JS are objects, and when you copy an object via newobj = origobj, you're just creating a reference. you need to CLONE the array object to create a truly independent copy.

Answer 1

Variable assignment in JavaScript never makes copies for non-primitives (everything that isn't a value).

Assignment for all non-values copies references.

Answer 2

A variable in javascript holds a reference to the array.

If you copy the variable value with arr2 = arr1, you copy the reference to the same array. So any change to arr2 is a change to arr1.

If you want another variable to hold a reference to a copy, so that you may change the second array without changing the first one, use slice :

var arr2 = arr1.slice();

• rodneyrehm: I don't remember why exactly, but I think you want that to be arr1.slice(0)
• rodneyrehm: You may also want to note that .slice() only creates a shallow copy of the array.