How to swap two variables in JavaScript

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

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>