Array flatten js function

index.js

// Of course we can use something like this [lodash flattenDeep](https://lodash.com/docs/4.17.15#flattenDeep)
// but here we want our solution

const nestedArray = [[1,2,[3]],4]
const moreNestedArray = [1,2,[[[5, 6, [8, [9], [10]]]]]]

const flat = array => {

  return array.reduce( (acc, cur) => 
    typeof cur === 'number'
      ? [...acc, cur]
      : [...acc, ...flat(cur)]
  , [] )

}

console.log( flat(nestedArray) )
console.log( flat(moreNestedArray) )