Which is faster: Math.abs(value) or value * -1 ?

Question

Pretty straightforward, but I just want to know which is faster.

I think simply multiplying a number by -1 is much faster than calling a predefined method, provided that you are sure that value is negative.

But if that's the case then what is the abs() function for? Is it simply for making sure that the value returned would always be positive regardless of value's sign?

Answer 1

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;