Updated August, 2012:

I did some profiling with these implementations:

/* Test 1: */ b = Math.abs(a);
/* Test 2: */ b = abs(a); //local copy: abs = Math.abs;
/* Test 3: */ b = a < 0 ? a * -1 : a;
/* Test 4: */ b = a < 0 ? -a : a;

I got the following result on Windows 7. Values are normalized after the fastest result per browser to make it easier to compare which method is faster:

        1:Math 2:abs 3:*-1  4:-    1.0=   Version
Chrome    1.0   1.0   1.0   1.0    111ms  21.0.1180.75 m
Firefox   1.0   1.0   1.2   1.2    127ms  14.0.1
IE        1.4   1.0   1.1   1.0    185ms  9.0.8112
Opera     1.9   1.6   1.1   1.0    246ms  12.00
Safari    1.6   1.6   1.1   1.0    308ms  5.1.7

Conclusion: When I did this test 3 years ago, -a was fastest, but now Math.abs(x) is faster in Firefox! In Chrome abs(a) and -a got the same time and it was only 3 ms difference to the slowest method when I tested it with 10 000 000 numbers.

My Recommendation: Use Math.abs(a). If you are in a tight loop and by profiling has found it to be too slow, you can use a local reference to the abs function:

var abs=Math.abs; //A local reference to the global Math.abs function
for (i=0;i<1234567890;++i) if ( abs( v[i] ) > 10) ++x;