Formatting hexadecimal Number to short UUID in JavaScript

Question

I have a hexadecimal number in javascript. For display purposes, I would like to format the string as:

ffffffff-ffff-ffff
00000000-0000-01ff

(8 digits)-(4 digits)-(4 digits) with padded zeros on the front

I've been trying to write my own loop to format an arbitrary hexadecimal number into this format, but this seems like something that should be available in JavaScript already.

Is there a built-in way to format a hexadecimal number in JavaScript?

Answer 1

I would do a two-step process:

1) convert number to 16 digit hex with leading zeros:

var i = 12345; // your number
var h = ("000000000000000" + i.toString(16)).substr(-16);

2) add dashes

var result = h.substr(0, 8)+'-'+h.substr(8,4)+'-'+h.substr(12,4);

Answer 2

Further to knabar's answer:

If your number is really a full 64 bits long you should be aware that javascript has only doubles, which top out at around 53 bits of precision. E.g.

var i = 0x89abcdef01234567; // a 64-bit constant
var h = ("000000000000000" + i.toString(16)).substr(-16); // "89abcdef01234800"

So you probably want to split this into two 32-bit numbers, and format them 8 digits at a time. Then the second caveat strikes: javascript performs bitwise ops on signed 32-bit integers, and this formatting code can't handle negative numbers.

var i = 0xffd2 << 16; // actually negative
var h = ("0000000" + i.toString(16)).substr(-8); // "0-2e0000"

Since it's fairly likely that numbers you want formatted in hexadecimal are the result of bitwise manipulations, the code can be tweaked to print in two's complement instead:

var i = 0xffd2 << 16; // actually negative
var h = ("0000000" + ((i|0)+4294967296).toString(16)).substr(-8); // "ffd20000"

This produces the hex representation of the bottom 32 bits of the integral part of arbitrary positive and negative numbers. This is probably what you want (it's approximately printf("%08x")). Some more corner cases:

var i = 1.5; // non-integers are rounded
var h = ("0000000" + ((i|0)+4294967296).toString(16)).substr(-8); // "00000001"

var i = -1.5; // rounding is towards zero
var h = ("0000000" + ((i|0)+4294967296).toString(16)).substr(-8); // "ffffffff"

var i = NaN; // not actually a number
var h = ("0000000" + ((i|0)+4294967296).toString(16)).substr(-8); // "00000000"

Answer 3

ES6 Version

function toPaddedHexString(num, len) {
    str = num.toString(16);
    return "0".repeat(len - str.length) + str;
}

var hexStr = toPaddedHexString(12345, 16);