JS Bin// source https://jsbin.com/fayomupale

flattenArray.js

const flattenArray = input => {
  if (!Array.isArray(input)) {
    throw new Error("Input must be an array")
  }
  
  let result = []
  for (let element of input) {
    if (Array.isArray(element)) {
      result = result.concat(flattenArray(element))
    } else {
      result.push(element)
    }
  }
  
  return result
}

const scenarios = {
  one: {
    input: [[1, 2, [3]], 4],
    expected: [1, 2, 3, 4],
  },
  two: {
    input: [1, [2], [], [3, [4, [5, [6, [], []]], 7], 8], 9, [], []],
    expected: [1, 2, 3, 4, 5, 6, 7, 8, 9],
  },
  three: {
    input: "ciao",
    expected: new Error("Input must be an array"),
  },
}

for (const [name, scenario] of Object.entries(scenarios)) {
  let actual = null
  try {
    actual = flattenArray(scenario.input)
  } catch (e) {
    actual = e
  }
  
  const jsonActual = JSON.stringify(actual)
  const jsonExpected = JSON.stringify(scenario.expected)
  if (jsonActual !== jsonExpected) {
    console.log(
      `Scenario ${name} failed, expected ${jsonExpected}, got ${jsonActual} instead.`)
  }
}

console.log("Done!")