flatten array

flatten-array.ts

interface NestedArray<T> extends Array<T | NestedArray<T>> { }

function flatten(arr: NestedArray<number>) {
  return arr.reduce((acc: number[], el) => {
    if (Array.isArray(el)) {
      return acc.concat(flatten(el))
    }
    return acc.concat(el)
  }, [])
}

function assertEqual(arr1: number[], arr2: number[]) {
  if (arr1.length !== arr2.length) {
    throw new Error("arrays length differ")
  }
  arr1.forEach((el, index) => {
    if (el !== arr2[index]) {
      throw new Error(`
        array elements are not equal: arr1[${index}] == ${el},
        arr2[${index}] == ${arr2[index]},
        ${el} !== ${arr2[index]}
      `)
    }
  })
}

function assertThrow(fn: Function, ...args: NestedArray<number>) {
  const NotThrownError = "expected function to throw"
  try {
    fn(...args)
    throw new Error(NotThrownError)
  } catch(e) {
    if (e.message == NotThrownError) {
        throw e
    }
  }
}

assertEqual(
  flatten([]),
  []
)
assertEqual(
  flatten([1]),
  [1]
)
assertEqual(
  flatten([1, [2]]),
  [1, 2]
)
assertEqual(
  flatten([[1,2,[3, [5, [6, [7], [8, [9]]]]]],4]),
  [1, 2, 3, 5, 6, 7, 8, 9, 4]
)
assertThrow(
  assertEqual,
  flatten([1, [2]]),
  [1, [2]],
)