js-arrays-functional.md

Javascript: Array Functional Methods

1. Determine what these functions do and give them a good name:

   const f1 = (n) => [...`${n}`].map((i) => parseInt(i));
   const f2 = (...nums) => nums.reduce((acc, val) => acc + val, 0) / nums.length;
   const f3 = (arr, val) => arr.reduce((a, v) => (v === val ? a + 1 : a), 0);
   const f4 = (A) => A.filter((item, index) => index === arr.indexOf(item));
   const f5 = (A) => [].concat(...A);

2. Write a function count that, given an array of numbers and a callback, counts the number of items for which the callback returns true.

3. Implement two functions, map and filter (receiving an array and a callback), in terms of forEach.

4. Likewise, implement find in terms of filter (ignore efficiency!).

5. Implement some and every in terms of map and filter.

6. Write a function singles that, given an array of numbers, returns a new array containing only the values of the original array that appear only once. Implement it in terms of filter.

7. Write a function uniq that, given an array of numbers, removes consecutive repetitions of values.

8. Implement the uniq function of the last exercise in terms of reduce.

9. Sort an array of objects such as

   {
     name: 'Fernando',
     lastname: 'Rodríguez',
     age: 39
   }

   by lastname

10. Write the findIndex method as a function

    function myFindIndex(array, fn) {
      /* ... */
    }

11. Write a function rotateArray(array, n) which rotates an array by n positions. For example: rotateArray([1, 2, 3, 4, 5], 2) will return [4, 5, 1, 2, 3].

12. Write a function which filters and array keeping only the elements that have the fields x and y.