flatten.ts

import assert from "assert";


function flatten<T>(array: (T|T[])[]): T[] {
  let flat: T[] = [];

  for (const el of array) {
    if (!Array.isArray(el)) {
      flat.push(el);
    } else {
      flat.push(...flatten(el));   
    }
  }

  return flat;
}

describe("flatten function", function() {
  it("returns empty on empty input", function() {
    assert.equal(flatten([]), []);
  });

  it("remains normal when given one level", function() {
    assert.equal(flatten([1, 2, 3]), [1, 2, 3]);
  });

  it("flattens a multi level array", function() {
    assert.equal(flatten([1, [2], 3]), [1, 2, 3]);
  });

  it("flattens a very deep complex array", function() {
    assert.equal(flatten([1, [[[[2, 3]]], 4], 5]), [1, 2, 3, 4, 5]);
  });

  it("works when given any type", function() {
    assert.equal(flatten(["a", ["b"], "c"]), ["a", "b", "c"]);
  });
});