How to swap two variables in JavaScript

I have this two variables:

var a = 1,
    b = 2;

My question is how to swap them? Only this variables, not any objects.

Here's a one-liner to swap the values of two variables.
Given variables a and b:

b = [a, a = b][0];

Demonstration below:

var a=1,
    b=2,
    output=document.getElementById('output');

output.innerHTML="<p>Original: "+a+", "+b+"</p>";

b = [a, a = b][0];

output.innerHTML+="<p>Swapped: "+a+", "+b+"</p>";

<div id="output"></div>

ES6 (Firefox and Chrome already support it (Destructuring Assignment Array Matching)):

let a = 5, b = 6;
[a, b] = [b, a];
console.log(`${a} ${b}`);

You can do this:

var a = 1,
    b = 2,
    tmp;
tmp = a;
a = b;
b = tmp;

For readability and maintainability, this can't be beat (at least in JavaScript). Anybody maintaining the code (including you six months from now) will know exactly what's going on.

Since these are integers, you can also use any number of clever tricks¹ to swap without using a third variable. For instance you can use the bitwise xor operator:

let a = 1, b = 2;
a = a ^ b;
b = a ^ b;
a = a ^ b;
    
console.log('a is now:', a);
console.log('b is now:', b);

This is called the XOR swap algorithm. Its theory of operation is described in this Wikipedia article.

¹"The competent programmer is fully aware of the limited size of his own skull. He therefore approaches his task with full humility, and avoids clever tricks like the plague." — Edsger W. Dijkstra

Don't use the code below. It is not the recommended way to swap the values of two variables (simply use a temporary variable for that). It just shows a JavaScript trick.

This solution uses no temporary variables, no arrays, only one addition, and it's fast. In fact, it is sometimes faster than a temporary variable on several platforms.
It works for all numbers, never overflows, and handles edge-cases such as Infinity and NaN.

a = b + (b=a, 0)

It works in two steps:

• (b=a, 0) sets b to the old value of a and yields 0
• a = b + 0 sets a to the old value of b

Since ES6, you can also swap variables more elegantly:

var a = 1,
    b = 2;

[a, b] = [b, a];

console.log('a:', a, 'b:', b); // a: 2 b: 1