How to left-shift numbers greater than 32-bits?

Question

As I understand it, JS treats numbers as 32-bits when performing bit-shift operations, even though it supports 64-bit numbers.

How can I implement a leftShift function that works on 64-bit numbers? i.e., it won't roll over into the negatives for something like 192 << 24 (should be 3221225472, not -1073741824).

Javascript numbers are 64-bit floating point, they only have 53 bits of mantissa. So there's no way to get 64-bit integers. — Barmar, Oct 14 '15 at 23:55
@Barmar That's OK. I just need 32 unsigned bits. 53 is plenty :D — mpen, Oct 14 '15 at 23:59

score 12 · Accepted Answer · edited Jan 27 '17 at 12:40

12

Just do what the math of a left shift does:

Arithmetic shifts are equivalent to multiplication by a positive (to the left) or by a negative number (to the right), integral power of the radix (e.g. a multiplication by a power of 2 for binary numbers).

function shift(number, shift) {
    return number * Math.pow(2, shift);
}

shift(192, 24)
//3221225472

shift(3221225472, -24)
//192

edited Jan 27 '17 at 12:40

David Gatti

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answered Oct 14 '15 at 23:47

dave

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Thank you. I thought it was that simple, but I wasn't sure if I missing something. – mpen Oct 15 '15 at 00:00
You should add a Math.floor() because of rounding errors. For example `0xFFFFFFFF * Math.pow(2, -24) = 255.99999994039536` instead of 255. – Motla Feb 15 '22 at 16:19

How to left-shift numbers greater than 32-bits?

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