Why do these statements work differently in JavaScript?

Question

I was recently checking out sweet little code to swap two variables using XOR function. I used JavaScript for this.

Let x and y be two variables and let x = 4, y = 6.

x = x ^ y; y = y ^ x; x = x ^ y;

It swaps variables nicely. Notice that I'm keeping x and y different to prevent aliasing which can occur due to first XOR.

Then, observing the statement, I wrote: x = x ^ (y = y ^ (x = x ^ y )); This swaps variable y correctly but makes x always 0.

Also, x ^= y ^= x ^= y; which also seems to be equivalent expression yields the same incorrect result as in second case.

However, all these statements run equivalently on Java and produce the same result consistently.

I also used strict mode with JavaScript.

Are these three statements somehow not equivalent on JavaScript or am I missing something critical here?

Answer 1

x = x ^ y;
y = y ^ x;
x = x ^ y;

This works because it's like

x_1 = x_0 ^ y_0;
y_1 = y_0 ^ x_1;
x_2 = x_1 ^ y_1;

Then,

x_final = x_2 = x_1 ^ y_1 = x_0 ^ y_0 ^ y_0 ^ x_1 = x_0 ^ y_0 ^ y_0 ^ x_0 ^ y_0
        = y_0
y_final = y_1 = y_0 ^ x_1 = y_0 ^ x_0 ^ y_0
        = x_0;

Your x = x ^ (y = y ^ (x = x ^ y )); is like

x_1 = x_0 ^ y_0;
y_1 = y_0 ^ x_1;
x_2 = x_0 ^ y_1;

Then,

x_final = x_2 = x_0 ^ y_1 = x_0 ^ y_0 ^ x_1 = x_0 ^ y_0 ^ x_0 ^ y_0
        = 0
y_final = y_1 = y_0 ^ x_1 = y_0 ^ x_0 ^ y_0
        = x_0;

It would have worked if you used x = (y = y ^ (x = x ^ y )) ^ x;.

That's because JS parses the expressions from left to right, and you want that x to be the modified value, not the initial one.