js實現展開一個數組

function flatten(arr) {
	let result = [];
	for (let i = 0; i < arr.length; i++) {
		if (Array.isArray(arr[i])) {
			result = result.concat(flatten(arr[i]));
		} else {
			result.push(arr[i]);
		}
	}
	return result;
}