Try
num.toString(2);
The 2 is the radix and can be any base between 2 and 36
source here
UPDATE:
This will only work for positive numbers, Javascript represents negative binary integers in two's-complement notation. I made this little function which should do the trick, I haven't tested it out properly:
function dec2Bin(dec)
{
if(dec >= 0) {
return dec.toString(2);
}
else {
/* Here you could represent the number in 2s compliment but this is not what
JS uses as its not sure how many bits are in your number range. There are
some suggestions httphttps://stackoverflow.com/questions/10936600/javascript-decimal-to-binary-64-bit
*/
return (~dec).toString(2);
}
}