flatten.js

function flatten(data) {
  return data.reduce(( acc, current ) => {
    if (Array.isArray(current)) {
      return acc.concat(flatten(current));
    } else {
      return acc.concat(current);
    }
  }, [])
}

const test = [[1,2,[3]],4];
const actual = flatten(test);

// Test Assertions

// Flattened length should equal 4
if (actual.length !== 4) {
  console.error('Failed: Length Not Eq 4');
}

// Flattened values should not contain array
for (const val of actual) {
  if (Array.isArray(val)) {
    console.error('Failed: Value is Array');
  }
}