index.js

export default function functionArguments(function_) {
	if (typeof function_ !== 'function') {
		throw new TypeError('Expected a function');
	}

	const commentRegex = /(\/\*([\s\S]*?)\*\/|([^:]|^)\/\/(.*)$)/gm;
	const quotes = ['`', '"', '\''];

	const functionSource = function_.toString().replace(commentRegex, ''); // Function with no comments

	let functionWithNoDefaults = '';
	let depth = 0; // () [] {}
	let index = 0;

	// To remove default values we can not use regexp because finite automaton can not handle such
	// things as (potential) infinity-nested blocks (), [], {}

	// Remove default values
	for (; index < functionSource.length && functionSource.charAt(index) !== ')'; index += 1) {
		// Exiting if an arrow occurs. Needed when arrow function without '()'.
		if (functionSource.startsWith('=>', index)) {
			functionWithNoDefaults = functionSource;
			index = functionSource.length;
			break;
		}

		// If we found a default value - skip it
		if (functionSource.charAt(index) === '=') {
			for (; index < functionSource.length && ((functionSource.charAt(index) !== ',' && functionSource.charAt(index) !== ')') || depth !== 0); index += 1) {
				// Skip all quotes
				let wasQuote = false;

				for (const quote of quotes) {
					if (functionSource.charAt(index) === quote) {
						index += 1;

						for (; index < functionSource.length && functionSource.charAt(index) !== quote;) {
							index += 1;
						}

						wasQuote = true;
						break;
					}
				}

				// If any quote type was skipped, start the cycle again
				if (wasQuote) {
					continue;
				}

				switch (functionSource.charAt(index)) { // Keeps correct depths of all types of parenthesises
					case '(':
					case '[':
					case '{':
						depth += 1;
						break;
					case ')':
					case ']':
					case '}':
						depth -= 1;
						break;
					default:
				}
			}

			if (functionSource.charAt(index) === ',') {
				functionWithNoDefaults += ',';
			}

			if (functionSource.charAt(index) === ')') { // Quits from the cycle immediately
				functionWithNoDefaults += ')';
				break;
			}
		} else {
			functionWithNoDefaults += functionSource.charAt(index);
		}
	}

	if (index < functionSource.length && functionSource.charAt(index) === ')') {
		functionWithNoDefaults += ')';
	}

	// The first part matches parens-less arrow functions
	// The second part matches the rest
	const regexFnArguments = /^(?:async)?([^=()]+)=|\(([^)]+)\)/;

	const match = regexFnArguments.exec(functionWithNoDefaults);

	return match ? (match[1] || match[2]).split(',').map(x => x.trim()).filter(Boolean) : [];
}