Flattern array in JS

flatten_array.js

function flatten_array( arr ){
	/**
	 * Checks if _arr is an array
	 * @param Array|Mixed _arr variable to check
	 * @return Bool   whether _arr is an array
	 */
	const is_array = ( _arr ) => {
		return typeof _arr === "object" && typeof _arr.concat === "function";
	};
	
	/**
	 * Return an error object if arr is not an Array
	 */
	if( !is_array( arr ) ){
		return new Error("Invalid argument provided " , arr , " should be an array" );
	}
	
	/**
	 * Reduces nested arrays into flattened array recursively
	 * 
	 * @param  Array res  result of previous loop
	 * @param  Array|Number item current item
	 * @return Array   Reduced array
	 */
	const _reducer = (res, item) => {
		if( is_array( item ) )
			return res.concat( item.reduce( _reducer, [] ) );
		else
			res.push(item);
		
		return res ;
	};
	
	/**
	 * Reduce arr to flattened array
	 */
	return arr.reduce( _reducer, [] );
}