flattenArray.js

// flattenArray([[1,2,[3]],4]) // [1, 2, 3 , 4]

function flattenArray (input) {
  if (Array.isArray(input)) {
    return input.reduce(
      function flattener (accumulator, value) {
        if (Array.isArray(value)) {
          return accumulator.concat(value.reduce(flattener, []))
        } else {
          return accumulator.concat(value)
        }
      }
      ,
      []
    )
  } else {
    throw new TypeError('Argument must be an array')
  }
}

// ES6 export
module.exports = exports.default = flattenArray