Code snippet to flatten a nested array

flattenArray.js

//A recurse function to flatten a nested array
function flatten(input, output = []) {
  input && input.forEach(element => {
		if (element instanceof Array) {
      //if found a array flatten it recursively
  		return flatten(element, output);
  	}
    //if non-array element add it the output
  	output.push(element);
	});
  return output;
}

function testFlatten() {
  if(!(JSON.stringify(flatten()) == JSON.stringify([]))) throw "Test case null fail";
  if(!(JSON.stringify(flatten([[1, 2, [3]], 4])) == JSON.stringify([1, 2, 3, 4]))) throw "Test case 1 fail";
}

testFlatten();