For the sake of brevity, here's the ugly one-liner version that's only slightly less ugly than all that concat and slicing above. The accepted answer is truly the way to go and way more readable.
Given:
var foo = [ 0, 1, 2, 3, 4, 5, 6 ];
if you want to swap the values of two indices (a and b); then this would do it:
foo.splice( a, 1, foo.splice(b,1,foo[a])[0] );
For example, if you want to swap the 3 and 5, you could do it this way:
foo.splice( 3, 1, foo.splice(5,1,foo[3])[0] );
or
foo.splice( 5, 1, foo.splice(3,1,foo[5])[0] );
Both yield the same result:
console.log( foo );
// => [ 0, 1, 2, 5, 4, 3, 6 ]
#splicehatersarepunks:)