// Note: This is coded in Javascript, ES6.

// Declare control variables.
const nest = [9, 8, [7, null, 6, [5, 4, [3, true], 2], 1, false, 'a'], 0, [-1, -2], [-3, ['b', -4, [-5, [-6, 'c']], -7], undefined, -8], -9];
let pure = [];

// Declare recursive function for checking item.
function checkItem(item) {
	// If item is array, check each item in array.
	if (Array.isArray(item)) {
		for (let i = 0; i < item.length; i++) {
			checkItem(item[i]);
		}
	}
	// If item is not a number, notify user.
	else if (isNaN(item) || item === undefined || item === null || item === true || item === false) {
		console.log(`\nItem is not a number: ${item}`);
	}
	// If item is number, append to array.
	else {
		pure.push(item);
	}
}

// Check nested array, then log results.
checkItem(nest);
console.log(`\nData: ${pure}`);