Decode HTML entities in TypeScript

decode-html-entities.ts

/**
 * A regular expression that matches HTML entities.
 *  - 1st Capture Group: The entire matched HTML entity.
 *  - 2nd Capture Group: The inner text of the entity (everything inside the '&' and ';' characters).
 *  - 3rd Capture Group: The integer value of the entity, if one is found (i.e. 'Ӓ' --> '1234'). 
 */
const HTML_ENTITY_REGEX: RegExp = /&(#([0-9]+)|[a-zA-Z0-9]+);/;

/**
 * Returns the input string after having converted all recognized HTML entities to their respective unicode characters.
 * 
 * @param {string} content The string (potentially) containing HTML entities to decode.
 * @returns {string} The input string after having converted all recognized HTML entities to their respective unicode
 * characters.
 */
function decodeHTMLEntities(content: string): string {
	
	/**
	 * If the given input string contains at least one recognized HTML entity, this function will return the unicode
	 * equivalent character for the aforementioned HTML entity. Otherwise, this function will simply return the original
	 * input.
	 * 
	 * @param {string} input The string HTML entity to decode to its unicode equivalent character.
	 * @returns {string} The unicode equivalent character of the first HTML entity found in the input string, or the
	 * original input string if no such HTML entity can be found.
	 */
	function decodeHTMLEntity(input: string): string {
		
		// Look for an HTML entity in the provided string.
		let entityMatch: RegExpMatchArray | null = input.match(HTML_ENTITY_REGEX);
		
		// If we didn't find an HTML entity in the provided input, just return the original string.
		if (entityMatch === null) return input;
		
		let entity: string = entityMatch[0];
		
		// Try the <textarea> trick to get JSDOM's virtual DOM to decode the HTML entity for us.
		let textArea: HTMLTextAreaElement = JSDOM.fragment("<textarea>").children[0] as HTMLTextAreaElement;
		textArea.innerHTML = entity;
		
		// The <textarea> trick successfully decoded the entity.
		if (textArea.value !== entity) return textArea.value;
		
		// The regex did not find an inner decimal value to use.
		if ((entityMatch[1] === undefined) && (entityMatch[2] === undefined)) return input;
		
		let parsedInt: number = parseInt(entityMatch[2] ?? entityMatch[1]);
		
		// We could not parse the value as an integer, and therefore cannot parse the HTML entity.
		if (isNaN(parsedInt)) return input;
		
		return String.fromCharCode(parsedInt);
		
	}
	
	return content.replace(new RegExp(HTML_ENTITY_REGEX, "g"), decodeHTMLEntity);
	
}