Why is this happening? (javaScript's reverse method)

Question

repl.it : https://repl.it/BuXR/3

var str = "abc";
var str2 = str.split(" ").join("").split("");
var tmp = str2;
console.log(str2);
// => ['a','b','c']
console.log(tmp.reverse());
// => ['c','b','a']
console.log(str2);
// => ['c','b','a']

My question is why str2 is being changed even though it is not being reversed?

This is very upsetting to me, but I have a guess as to why this is happening. The tmp is just a pointer to the original str2, and when I call reverse() on tmp, it actually reverses str2.

Even if that really is what's happening, I still feel like it is a very counterintuitive way for a language to work.

Answer 1

Your guess is right.

The tmp is just a pointer to the original str2, so whatever operations are performed on str2 it will be stored to memory and when you access tmp it find the reference to str2.

I have a guess as to why this is happening. The tmp is just a pointer to the original str2, and when I call reverse() on tmp, it actually reverses str2.

Answer 2

2 Reasons why this is happening:

• The reverse() method reverses an array in place, hence tmp is reversed: ref
• The variable tmp and str2 are references to the same array instance, hence str2 also reversed.