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flatMap-flat.spec.js

import {strict as assert} from 'assert';

describe('flat()', () => {
  it('an array of arrays returns a flat array', () => {
    const twoDimensionalArray = [['a'], ['b'], ['c']];
    const flattenedArray = twoDimensionalArray.flat();
    assert.deepEqual(flattenedArray, ['a', 'b', 'c']);
  });
  it('flat() without parameter flattens an n-dimensional array into a n-1-dimensional array', () => {
    const threeDimensionalArray = [[['a']]];
    assert.deepEqual(threeDimensionalArray.flat(), [['a']]);
  });
  it('flat(X) flattens an n dimensional array into an n-X dimensional array', () => {
    const threeDimensionalArray = [[['a']]];
    assert.deepEqual(threeDimensionalArray.flat(2), ['a']);
  });
  it('flat() returns the same thing as flat(1)', () => {
    const flattenedWithoutDepthParameter = [[['a']]].flat();
    const flattenedWithOne = [[['a']]].flat(1);
    assert.deepEqual(flattenedWithOne, flattenedWithoutDepthParameter);
  });
  it('flat(0) returns a copy of the array', () => {
    const a = ['a'];
    const b = a.flat(0);
    assert.notEqual(a, b, 'Should not be the same object');
    assert.deepEqual(a, b, 'Should have the same values');
  });

  it('flat(N) of an array of depth smaller then N flattens to a one dimensional array', () => {
    const threeDimensionalArray = [[['a']]];
    const flattened = threeDimensionalArray.flat(4);
    assert.deepEqual(flattened, ['a']);
  });

  describe('flat() with a string as parameter', () => {
    it('where the string can be converted to a number, flattening works as with a number', () => {
      const threeDimensionalArray = [[['a']]];
      const flattened = threeDimensionalArray.flat('1');
      assert.deepEqual(flattened, [['a']]);
    });
    it('where the string is not numerical, flattening uses 0 as argument', () => {
      const threeDimensionalArray = [[['a']]];
      const flattened = threeDimensionalArray.flat(' a ');
      assert.deepEqual(flattened, threeDimensionalArray);
    });
  });
});

describe('flatMap()', () => {
  it('the result of flatMap() is equivalent to the result map().flat()', () => {
    const threeDimensionalArray = [[['a']]];
    const identity = x => x;
    assert.deepEqual(
      threeDimensionalArray.flatMap(identity),
      threeDimensionalArray.map(identity).flat()
    );
  });
});