I've written a lot of Javascript over the past few years. How much? I'm not sure, but 100 thousand lines is probably a good estimate^\[0\]^^\[1\]^. The two functions  that I've written the most, over and over (and over and over and over... and over), are no doubt the function to convert hexadecimals to bytes, and vice versa.

Part of this is because I do a lot of stuff related to cryptography and cryptocurrency (which, no surprise, is basically just *more* cryptography), which involves tons of work with bytes and often, converting them to hex for storage or display. The other part is because Javascript doesn't have a builtin way to convert hex to bytes or the other way around (Node.js apparently has `Buffer.from` but I never use that), and also because I just like writing things from scratch, which you may notice is a common theme in this blog. In addition to my trademark unnecessarily long sentences, of course.

## Bytes

I assume you already know what bytes and hexadecimals are, but in case you don't, here's a brief overview.

Bits have two states. Bytes are made out of eight bits, so one byte can have 256 (`2^8=256`) states.

Now, there are a couple ways you can represent bytes. One way could be representing them in binary, with 1s and 0s. Another would be just using our normal decimal (base 10 numbers), where a byte could be represented by a number from 0 to 255. But the best way (in my opinion) is to use hexadecimals (base 16 numbers) which uses the digits 0-9 and A-F. `A` represents 10, `B` represents 11, and so on. `FF` would represent 255 in decimal (`15*16+15=255`), `10` would represent 16 (`1*16+0`), and `32` would represent 50 (`3*16+2=50`).

Why base 16? If you remember, one byte can have 256 states, meaning that two hexadecimal digits can perfectly represent one byte (`16^2=256`) which is a lot more elegant than decimal, and a lot more concise than binary. A base 256 would of course, be impractical. With decimal, it isn't exactly clear how many bytes 2402655566 is, while it is very clear how many bytes 8F359D4E is (8 hex digits, so 4 bytes).

## Uint8Array

Anyways, back on topic. In Javascript, bytes are often represented by [Uint8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array), which are shockingly an array of Uint8s. What are Uint8s? Uint means "unsigned integer", or basically a non-negative whole number. The 8 stands for the 8 bits, so a Uint8 is an array of one byte unsigned integers^\[2\]^. Basically, it's a way to represent bytes in Javascript by storing in as an array of decimal numbers from 0-255.

## Converting Bytes to Hexadecimal

```js
function uint8_to_hex(uint8) {
  const hex_chars = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'];
  let hex = "";
  for (let i=0; i < uint8.length; i++) {
    hex += hex_chars[Math.floor(uint8[i]/16)];
    hex += hex_chars[uint8[i] % 16];
  }
  return hex;
}
```

The loop iterates through through the `Uint8Array`, first dividing it by 16 and rounding down, to find the first hex character. Then, it divides by 16 and takes the remainder for the second hex character.

## Converting Hexadecimal to Bytes

```js
function hex_to_uint8(hex) {
  const hex_chars = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'];
  hex = hex.toUpperCase();
  let uint8 = new Uint8Array(Math.floor(hex.length/2));
  for (let i=0; i < Math.floor(hex.length/2); i++) {
    uint8[i] = hex_chars.indexOf(hex[i*2])*16;
    uint8[i] += hex_chars.indexOf(hex[i*2+1]);
  }
  return uint8;
}
```

Here, we determine how many whole bytes^\[4\]^ are in the hex string, by diving it by two. We then loop that many times, each time convering the two hex characters into a number by finding the value of the first hex character (`indexOf`), multiplying it by 16, then finding the value of the second of the second hex character, and adding it.

By the way, doing, for example, `new Uint8Array(5)`, will initialize an `Uint8Array` of all 0s, of length 5.

This function, as written, isn't designed to take in invalid input, so make sure to validate any inputs. In fact, I would encourage you to go and write your own conversion functions, instead of copy pasting these examples. You'll (hopefully) understand the concepts much faster that way.

===
- \[0\]: ±50 thousand lines (estimating skills are not my strong suit).
- \[1\]: [my 6000 lines of unfinished code in one horrific file](https://github.com/jetstream0/Muskets-and-Bayonets/blob/main/script.js).
- \[2\]: Signed integers have "signs", ie, they can represent negative numbers.
- \[3\]: In case you were wondering, I do write the `hex_chars` array out every time... slightly painful, but it's too much work to copy paste it from somewhere
- \[4\]: Note that the `Math.floor` means that this function only works with an even hex string length, since an odd hex string length would mean there's half of a byte (aka a nybble) being used, which is rare-ish.
