flatten-array.ts

// Two files (the helper function and its associated tests) have been
// concatenated here for the sake of fitting into one Gist.

// File: flatten-array.ts
// ----------------------
// Type definitions
type Nested<T> = Array<T | Nested<T>>

// Flattens an array by recursively iterating through items of the
// (nested) array, returning the values at the leaf nodes.
const flattenArray = (arr: Nested<number>) =>
  arr.reduce<Array<number>>(
    (flat, value) =>
      flat.concat(Array.isArray(value) ? flattenArray(value) : value),
    []
  )

// Exports
export { flattenArray }


// File: flatten-array.test.ts
// ---------------------------
// Dependencies
import * as assert from 'assert'
import { flattenArray } from './flattenArray'

// Makeshift test suite
try {
  // Tests
  assert.deepEqual(flattenArray([]), [], 'Empty array')
  assert.deepEqual(flattenArray([[], [[], [[], []]]]), [], 'Empty nested array')
  assert.deepEqual(flattenArray([1, 2, 3, 4]), [1, 2, 3, 4], 'Identity')
  assert.deepEqual(flattenArray([[1, 2, [3]], 4]), [1, 2, 3, 4], 'Nested array')

  console.log('Tests passed!')
} catch (err) {
  // Assertion error logging
  if (err instanceof assert.AssertionError) {
    const { message, expected, actual } = err

    console.error(
      `Test: ${message}\nExpected: ${JSON.stringify(
        expected
      )}\nActual: ${JSON.stringify(actual)}`
    )
  } else {
    console.error(err)
  }
}