Simple Array flatten implementation in js

flatten.js

// Takes an array with possible nested arrays, and returns a **new** flat array.
const flatten = array =>
	array.reduce(
		(flattened, elem) =>
			flattened.concat(Array.isArray(elem) ? flatten(elem) : elem),
		[]
	);

// From here on below we have only test utilities
const test = (testedFunc, input, expectedOutput, comparator) => {
	const output = testedFunc(input);
	const testSuccess = comparator(output, expectedOutput);

	if (!testSuccess)
		throw new Error(`Test unsuccessful, expected ${JSON.stringify(expectedOutput)}, got ${JSON.stringify(output)}`);
  
	return true;
};

// Shallow comparison between arrays.
const arrayEquals = (array1, array2) =>
	array1.length === array2.length &&
	array1.every((elem, index) => elem === array2[index]);

const runTests = () => {
	try {
		// Returns equal array on flat arrays
		test(flatten, [1, 2, 3], [1, 2, 3], arrayEquals);

		// Flattens array with a singleton array
		test(flatten, [1, 2, [3]], [1, 2, 3], arrayEquals);

		// Flattens array with nested array with multiple values
		test(flatten, [1, [2, 3]], [1, 2, 3], arrayEquals);

		// Works on edge cases
		test(flatten, [[]], [], arrayEquals);
		test(flatten, [], [], arrayEquals);
		test(flatten, [[], []], [], arrayEquals);
		test(flatten, [[[]]], [], arrayEquals);
	} catch (error) {
		console.log(error);
	}
};

runTests(); // OK