Increase type coverage and documentation for StringXor class.

lib/util/StringXor.js

@@ -5,19 +5,57 @@
 
 "use strict";
 
+/** @typedef {import("../util/Hash")} Hash */
+
+/**
+ * StringXor class provides methods for performing
+ * [XOR operations](https://en.wikipedia.org/wiki/Exclusive_or) on strings. In this context
+ * we operating on the character codes of two strings, which are represented as
+ * [Buffer](https://nodejs.org/api/buffer.html) objects.
+ *
+ * We use [StringXor in webpack](https://github.com/webpack/webpack/commit/41a8e2ea483a544c4ccd3e6217bdfb80daffca39)
+ * to create a hash of the current state of the compilation. By XOR'ing the Module hashes, it
+ * doesn't matter if the Module hashes are sorted or not. This is useful because it allows us to avoid sorting the
+ * Module hashes.
+ *
+ * @example
+ * ```js
+ * const xor = new StringXor();
+ * xor.add('hello');
+ * xor.add('world');
+ * console.log(xor.toString());
+ * ```
+ *
+ * @example
+ * ```js
+ * const xor = new StringXor();
+ * xor.add('foo');
+ * xor.add('bar');
+ * const hash = createHash('sha256');
+ * hash.update(xor.toString());
+ * console.log(hash.digest('hex'));
+ * ```
+ */
 class StringXor {
 	constructor() {
+		/** @type {Buffer|undefined} */
 		this._value = undefined;
 	}
 
 	/**
+	 * Adds a string to the current StringXor object.
+	 *
 	 * @param {string} str string
 	 * @returns {void}
 	 */
 	add(str) {
 		const len = str.length;
 		const value = this._value;
 		if (value === undefined) {
+			/**
+			 * We are choosing to use Buffer.allocUnsafe() because it is often faster than Buffer.alloc() because
+			 * it allocates a new buffer of the specified size without initializing the memory.
+			 */
 			const newValue = (this._value = Buffer.allocUnsafe(len));
 			for (let i = 0; i < len; i++) {
 				newValue[i] = str.charCodeAt(i);
@@ -41,11 +79,24 @@ class StringXor {
 		}
 	}
 
+	/**
+	 * Returns a string that represents the current state of the StringXor object. We chose to use "latin1" encoding
+	 * here because "latin1" encoding is a single-byte encoding that can represent all characters in the
+	 * [ISO-8859-1 character set](https://en.wikipedia.org/wiki/ISO/IEC_8859-1). This is useful when working
+	 * with binary data that needs to be represented as a string.
+	 *
+	 * @returns {string} Returns a string that represents the current state of the StringXor object.
+	 */
 	toString() {
 		const value = this._value;
 		return value === undefined ? "" : value.toString("latin1");
 	}
 
+	/**
+	 * Updates the hash with the current state of the StringXor object.
+	 *
+	 * @param {Hash} hash Hash instance
+	 */
 	updateHash(hash) {
 		const value = this._value;
 		if (value !== undefined) hash.update(value);