array_flatten.js

// input [[1,2,[3]],4] -> [1,2,3,4]

/**
 * Assumptions
 * 1. We don't need to take care of duplicates.
 * 2. Arrays are only interger values.
 * 3. No null or undefined values allowed.
 */

function run(input) {
  const flatten = (arr) => arr.reduce((acc, item) => {
    if (Array.isArray(item) && item.length > 0) {
      acc = acc.concat(flatten(item));
    } else {
      acc.push(item);
    }
    return acc;
  }, []);
  
  return flatten(input);
}

function equals(actual, expected) {
  const a = JSON.stringify(actual);
  const b = JSON.stringify(expected);
  console.log(a, b);
  return a === b;
}

let actual;

actual = run([1,2,[3],4]);
console.log(equals(actual, [1, 2, 3, 4]));

actual = run([[1,2,[3]],4]);
console.log(equals(actual, [1, 2, 3, 4]));

actual = run([[1,2,[3]],[4]]);
console.log(equals(actual, [1, 2, 3, 4]));