Flatten an array in TypeScript

README.md

Flatten an array in TypeScript

Working code: https://codesandbox.io/s/ancient-snowflake-b8keo

flatten.test.ts

import { flatten, flattenShort } from "./flatten";

describe("flatten tests (iterative implementation)", () => {
  it("should work well with flat arrays", () => {
    const actual = flatten([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
    const expected = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    expect(actual).toEqual(expected);
  });

  it("should flatten arbitrary nested arrays", () => {
    const actual = flatten([[1, 2, [3]], 4]);
    const expected = [1, 2, 3, 4];

    expect(actual).toEqual(expected);
  });
});

describe("flattenShort tests (recursive implementation)", () => {
  it("should work well with flat arrays", () => {
    const actual = flattenShort([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
    const expected = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    expect(actual).toEqual(expected);
  });

  it("should flatten arbitrary nested arrays", () => {
    const actual = flattenShort([[1, 2, [3]], 4]);
    const expected = [1, 2, 3, 4];

    expect(actual).toEqual(expected);
  });
});

flatten.ts

/* Once TypeScript releases recursive types https://github.com/microsoft/TypeScript/pull/33050
  we could implement the NestedIntegerArray as follows:

  export type NestedIntegerArray = Array<number | NestedIntegerArray>;
*/
export type NestedIntegerArray = Array<any>;

/**
 * Flatten an array of arbitrarily nested arrays of integers into a flat array of integers (iterative implementation).
 *
 * @typedef {Array<number|NestedIntegerArray>} NestedIntegerArray
 * @param {NestedIntegerArray} arr - An array of arbitrarily nested arrays of integers
 * @return {Array<number>} A flat array of integers
 *
 * @example
 *
 *     flatten([[1,2,[3]],4]) -> [1,2,3,4]
 */
export function flatten(arr: NestedIntegerArray): Array<number> {
  let result = [];
  let stack = [arr];
  let indexes = [0];

  while (stack.length > 0) {
    let current = stack.pop();
    let currentIndex = indexes.pop();
    for (let i = currentIndex, len = current.length; i < len; i++) {
      let element = current[i];
      if (Array.isArray(element)) {
        stack.push(current, element);
        indexes.push(i + 1, 0);
        break;
      } else {
        result.push(element);
      }
    }
  }

  return result;
}

/**
 * Flatten an array of arbitrarily nested arrays of integers into a flat array of integers (recursive implementation).
 *
 * @typedef {Array<number|NestedIntegerArray>} NestedIntegerArray
 * @param {NestedIntegerArray} arr - An array of arbitrarily nested arrays of integers
 * @return {Array<number>} A flat array of integers
 *
 * @example
 *
 *     flattenShort([[1,2,[3]],4]) -> [1,2,3,4]
 */
export function flattenShort(arr: NestedIntegerArray): Array<number> {
  return arr.reduce(
    (acc: Array<number>, val) =>
      acc.concat(Array.isArray(val) ? flattenShort(val) : val),
    []
  ) as Array<number>;
}