Flatten a nested array to a single level array

nestedArray.js

// Setup nested array
var array1 = [[1,2,[3]],4];

// Iterate over the nested array and reduce/concat
function flattenArray(arr1) {
  return arr1.reduce((acc, val) => Array.isArray(val) ? acc.concat(flattenArray(val)) : acc.concat(val), []);
}

// Run the nested array through our function
flattenArray(array1);
// => [1, 2, 3, 4]