Extract bits from start to end in javascript

Question

In Java script I want to extract bits 13 to 16 from integer number. Example: If I extract bits 13 to 16 from number 16640 then output will be 2

I have searched on google and found few links but they are in C language.

Answer 1

Assuming your bit count starts at 0:

var extracted, orig;

orig = parseInt("16640", 10); // best practice on using parseInt: specify number base to avoid spurious octal interpretation on leading zeroes (thx Ken Fyrstenberg)
extracted = ((orig & ((1 << 16) - 1) & ~(((1 << 13) - 1))) >>> 13);

Explanation:

• mask the lower 16 bits of the original number
• mask the complement of the lower 13 bits of the result (ie. bits 13-31)
• you currently have bits 13-16 of the orignal number in their original position. shift this bit pattern 13 bits to the right.

Note that this method only works reliably for numbers less than 2^31. The docs (MDN) are here

Answer 2

Javascript's bitwise operations work essentially the same way as they do in C:

var bits13to16 = (number >> 13) & 15;

This shifts the number 13 bits to the right (eliminating bits 0-12) and masks all but the last 4 remaining bits (which used to be bits 13-16). 15 = 2^4 - 1.

Answer 3

All suggestions are working but the simplest I think is given by @dandavis.

parseInt( 16640 .toString(2).slice(-16, -13), 2 );