Let's Polyfill - map(), filter() and reduce()

The hello world of Polyfills

Map

map is an Array method that takes in a callback and returns an array of items that were returned from the callback

Example:

1const arr = [1, 2, 3, 4];
2const res = arr.map((el) => el * 2);
3
4console.log(res); // returns [2,4,6,8]

Let’s create our own map method called myMap

  • myMap() takes in a parameter which a callback/function.

  • It has a results array that gets returned by the myMap function.

  • The returned values from our cb are pushed in the results array.

  • The this here would be the array that we will use this myMap function on.

  • The traditional map() callback can take 3 args. element, index and the source arr. We have done the same.

1function myMap(cb) {
2  // rseults results array that gets returned at the end
3  const results = [];
4
5  for (let i = 0; i < this.length; i++) {
6    // returned values of our cb are pushed in the reults[]
7    // 'this' referes to the passed array
8    results.push(cb(this[i], i, this));
9  }
10
11  return results;
12}
13
14// Doing this will allow us to use arr.myMap() syntax
15Array.prototype.myMap = myMap;
16
17const arr = [1, 2, 3, 4, 5, 6];
18const myMapResult = arr.myMap((el, _idx, _arr) => {
19  return el * 2;
20});
21
22console.log(myMapResult); //[2, 4, 6, 8, 10, 12];

Filter

filter() is an Array method that takes in a callback and returns an array of items that satisfy the condition provided in our callback

Example:

1const arr = [1, 2, 3, 4];
2const res = arr.filter((el) => el % 2); // only return even numbers
3
4console.log(res); // [2,4]

Let’s create our own filter method called myFilter

  • myFilter() takes in a parameter which a callback/function.

  • It has a results array that gets returned at the end.

  • The returned values from our cb are pushed in the results array.

  • The this here would be the array that we will use this myFilter function on.

  • The traditional filter() callback can take 3 args. element, index and the source arr. We have done the same.

1function myFilter(cb) {
2  const results = [];
3
4  for (let i = 0; i < this.length; i++) {
5    const cbResult = cb(this[i], i, this);
6    // the returned value of callback is true only then push it to the results
7    if (cbResult) results.push(this[i]);
8  }
9
10  return results;
11}
12
13// Doing this will allow us to use arr.myFilter() syntax
14Array.prototype.myFilter = myFilter;
15
16const arr = [1, 2, 3, 4, 5, 6];
17
18const foo = [
19  { name: "S", age: 2 },
20  { name: "V", age: 3 },
21];
22
23const myFilterResult = foo.myFilter((el, _idx, _arr) => {
24  return el.name !== "S";
25});
26
27console.log(myFilterResult); // [{ name: "V", age: 3 }]

Reduce

Here the MDN definition of it.

The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in a single output value.

It takes in two important parameters. accumulater and currentValue

Example:

1const arr = [1, 2, 3, 4];
2const res = arr.reduce((acc, curr) => {
3  acc += curr;
4  return acc;
5}); // 10
6
7console.log(res); // 10

Lets create our own reduce() method called myReduce()

  • myReduce() takes in a parameter which a callback/function.

  • It returns a single reduced value.

  • The returned values from our cb is assigned to the acc.

  • The this here would be the array that we will use this myReduced function on.

  • The traditional reduced() callback can take 4 args. accumulator, currentValue, index and the source arr. We have done the same.

1function myReduce(cb, initialValue) {
2  let acc;
3  let curr;
4
5  if (!this.length && !initialValue)
6    throw new Error("Can't reduce on empty array, provide initial value");
7  else {
8    //  If initialValue is given then acc is that or acc = is the 0th index of this
9    acc = initialValue ? initialValue : this[0];
10    for (let i = 1; i < this.length; i++) {
11      // current value of the array
12      curr = this[i];
13      // the retuned cb value is assigned to acc
14      acc = cb(acc, curr, i, this);
15    }
16  }
17  return acc;
18}
19
20// Doing this will allow us to use arr.myReduce() syntax
21Array.prototype.myReduce = myReduce;
22
23const myReduceResult = arr.myReduce((acc, curr, _idx, _arr) => {
24  acc += curr;
25  return acc;
26});
27
28console.log(myReduceResult); // 21

If you find any errors or edge cases in the above code then please let me know. I am happy to learn about them and add them here.

In the next blog in this series, I’ll try and write our own debounce function from the loadash library

Also, if you guys want to see polyfills of your libs then let me know in the comments.

Hope this blog was helpful to you.