How to Convert Between Hexadecimal Strings and Byte Arrays in JavaScript

Introduction:

As a junior to mid-level developer, it's important to understand the basics of converting between hexadecimal strings and byte arrays in JavaScript. This knowledge can come in handy when working with HTML forms, random numbers, Bloom filters, and other applications where a hexadecimal representation of data is required.

Hexadecimal Numbers:

Hexadecimal is a number system that uses 16 symbols (0-9 and A-F) to represent values. It's often used to represent a string of binary data as a string of hexadecimal characters. This representation is compact and easy to read, making it a popular choice for many applications.

Function bytestohex:

The function bytestohex takes a byte array as its input and returns a hexadecimal string. The first step in converting a byte array to a hexadecimal string is to create a new Uint8Array from the input byte array. This is done using the following code:

const bytes = new Uint8Array(inputByteArray);

Next, we'll use a loop to iterate over each byte in the array, converting each one to a hexadecimal string. This is done using the following code:

function bytestohex(bytes) {
    var hex = '';
    for (var i = 0; i < bytes.length; i++) {
        hex += ('0' + (bytes[i] & 0xFF).toString(16)).slice(-2);
    }
    return hex;
}

Function stringtoutf:

The function stringtoutf takes a hexadecimal string as its input and returns a byte array. The first step in converting a hexadecimal string to a byte array is to use the TextEncoder object to create a new instance of a TextEncoder. This is done using the following code:

const encoder = new TextEncoder();

Next, we'll use the TextEncoder's encode method to convert the hexadecimal string to a byte array. This is done using the following code:

function stringtoutf(hex) {
    var arr = [];
    for (var i = 0; i < hex.length; i += 2) {
        arr.push(parseInt(hex.substr(i, 2), 16));
    }
    return new Uint8Array(arr);
}

Using a DataView:

In some cases, you may need to convert a hexadecimal string to a byte array, and then back to a hexadecimal string, while preserving the original hexadecimal value. One way to do this is to use a DataView object.

const view = new DataView(new ArrayBuffer(4));
const codepoints = [0x41, 0x42, 0x43, 0x44];
for (var i = 0; i < codepoints.length; i++) {
    view.setUint8(i, codepoints[i]);
}

UTF-16 Code-Units:

It's important to note that JavaScript uses UTF-16 encoding for its strings, which means that each character in a JavaScript string is represented by two code-units. When converting a hexadecimal string to a byte array, you need to be mindful of the UTF-16 encoding, as the resulting byte array may contain more bytes than expected if the string contains characters outside of the basic Latin alphabet.

Endian Order and Endianess Issues:

Another important consideration when converting between hexadecimal strings and byte arrays is endian order. Endian order refers to the order in which the bytes of a multi-byte value are stored in memory. There are two main types of endian order: little-endian and big-endian.

When converting a hexadecimal string to a byte array, you need to be mindful of endianess issues, as the byte order of the resulting byte array may not match the byte order of the hexadecimal string. This can result in unexpected behavior when working with the byte array.

Even Weirder Cases:

In some cases, you may need to convert a hexadecimal string to a JavaScript string, where each character in the string represents a hexadecimal value. To do this, you can use regular expressions to replace each pair of hexadecimal characters in the string with its corresponding UTF-16 code-unit.

function converthextostring(hex) {
    var str = '';
    for (var i = 0; i < hex.length; i += 2) {
        str += String.fromCharCode(parseInt(hex.substr(i, 2), 16));
    }
    return str;
}

Conclusion:

In conclusion, converting between hexadecimal strings and byte arrays in JavaScript can be a challenging task, particularly when working with UTF-16 code-units and endianess issues. However, by understanding the basics of hexadecimal numbers and using functions such as bytestohex, stringtoutf, and converthextostring, you can convert data with ease. Additionally, using a JavaScript library, such as the hex converter in the JavaScript library, can simplify the process even further.