Flatten array sample assuming input will only ever contain nested arrays of numbers

flattenArray.js

function flatten(arr, flat) {

  let output = !!flat ? flat : []

  arr.forEach((el) => {
    if (typeof el === 'number') {
      output.push(el)
    } else {
      flatten(el, output)
    }
  })

  return output

}

flatten([1, 2, [3, [4], 5, 6], 7]) // [1, 2, 3, 4, 5, 6, 7]