Way to add leading zeroes to binary string in JavaScript

Question

I've used .toString(2) to convert an integer to a binary, but it returns a binary only as long as it needs to be (i.e. first bit is a 1).

So where:

num = 2;
num.toString(2) // yields 10.

How do I yield the octet 00000010?

You could just add zeroes on the front of the result until it is the correct length. — forgivenson, Dec 24 '14 at 20:20
Answered here http://stackoverflow.com/questions/24337260/javascript-a-byte-is-suppose-to-be-8-bits — sarvesh, Dec 24 '14 at 20:22

Joe Thomas · Accepted Answer · 2018-02-06T01:05:32.217

40

It's as simple as

var n = num.toString(2);
n = "00000000".substr(n.length) + n;

edited Feb 06 '18 at 01:05

answered Dec 24 '14 at 20:20

Joe Thomas

13

Now you can use standard 'n.padStart(paddingSize, '0');' – DevMultiTech Dec 05 '19 at 09:34
@DevMultiTech, you should make yours an answer and it should be the chosen one. function pad(v,s=8){return v.toString(2).padStart(s, '0'); 1. pad(1); /* '00000001' */ 2. pad(1,2); /* '01' */ – Beez Aug 21 '21 at 00:23

score 6 · Answer 2 · edited Apr 30 '21 at 17:35

6

You could just use a while loop to add zeroes on the front of the result until it is the correct length.

var num = 2,
    binaryStr = num.toString(2);

while(binaryStr.length < 8) {
    binaryStr = "0" + binaryStr;
}

edited Apr 30 '21 at 17:35

Jett

answered Dec 24 '14 at 20:23

forgivenson

score 2 · Answer 3 · answered Dec 24 '14 at 20:20

2

Try something like this ...

function pad(n, width, z) {
  z = z || '0';
  n = n + '';
  return n.length >= width ? n : new Array(width - n.length + 1).join(z) + n;
}

... then use it as ...

pad(num.toString(2), 8);

answered Dec 24 '14 at 20:20

rfornal

3 Answers3