trainbolt

const n = [1,[2,[3,4,5]],6,[7,[8],[9,[10]]]];
const n2 = [1,[2,[3,[4,[5],6],7],8],9];

/**
 * Flattens arbitrarilily nested arrays. Pulls off the first item (a) from the array
 * and preserves the rest (arr) using the spread operator. 
 * 
 * @param {Array} [] The nested array to flatten
 * @param {Array} flat The nested array to flatten
 * @returns {Array} Recursively returns a flattened array