Polyfills for JavaScript: A Full Overview
Tags: javascript, polyfills, webdev, frontend
Published: 2023-04-16 21:31:57

## Overview:

You may be familiar with the phrase `"polyfill"` as a developer. In a continuously changing technological landscape where new capabilities are frequently added to programming languages and online platforms, polyfills are an essential component of web development. Since not all browsers and environments immediately support the newest features, it can be difficult for developers to stay up to date with these changes.

A `polyfill` is a piece of code that enables the usage of new programming language or web platform features in `outdated browsers` or environments that do not support them. We will look deeply into JavaScript polyfills in this blog article, discussing their advantages and how to use them in your code.

---

## The Need for Polyfills

1. **New features**: Polyfills enable programmers to use `cutting-edge functions` of a web platform or programming language in older browsers or environments that don't support them.

2. **Browser Compatibility**: The latest features of a programming language or web platform may not be supported by all browsers and environments, which can cause compatibility problems for developers. Polyfills aid in ensuring that the code functions consistently on various browsers and gadgets.

---

## Benefits of Using Polyfills


1. **Better User Experience**: Polyfills allow developers to improve `user experience` by adding features and capabilities, even in older browsers.

2. **Reduced development time**: By enabling developers to use new capabilities without having to write specific code for `older browsers`, polyfills can save developers a significant amount of effort.

3. **Cost-effective**: Using polyfills saves money by preventing the need to design specialised solutions for every browser or device by ensuring that your code works on a variety of `browsers and mobile devices`.

4. **Enhanced security**: Using polyfills can also increase `security` by offering a different implementation of a feature that might have security flaws in `earlier browser versions`.

5. **Better cross-browser compatibility**: Polyfills enable programmers to use new capabilities of a `programming language` or `web platform` in situations or environments that don't support them, ensuring that their code functions reliably across various `browsers and devices`.

6. **Future-proofing**: By incorporating new capabilities that might become standards in upcoming iterations of the programming language or web platform, developers can `future-proof` their code by utilising polyfills.

7. **Open-source community**: Developers can use a variety of `open-source` polyfill libraries to take use of the skills and knowledge of other developers.

8. **Better accessibility**: By offering fallbacks for features that may not be supported by `assistive technologies`, polyfills can help make web applications more accessible to users with impairments.

---

## Implementing Polyfills in JavaScript

The method of implementing polyfills in `JavaScript` is simple. In this part, we'll examine a couple of code examples that demonstrate how to use polyfills. We will go over the steps for putting polyfills into JavaScript in this section:


**Step 1: Identify the missing feature**

- Finding the feature that is absent from the `browser or environment` is the first step in constructing a polyfill. 
- This can be accomplished by determining whether the feature is compatible with the browser or by determining whether it is present using feature detection methods like `typeof`, `in`, or `window`.


**Step 2: Write the fallback implementation**

- The next step is to develop the fallback implementation that offers a similar functionality once the missing feature has been located. 
- A pre-existing `polyfill library` or pure `JavaScript code` can both be used to do this.


**Step 3: Examine the polyfill**

- It is crucial to properly test the polyfill after implementation to make sure it `functions` as intended. 
- To check that the code provides the same functionality across various contexts and `browsers`, you can test it.


**Step 4: Implement the polyfill in your code**

- Implementing the polyfill in your code comes after `testing` it. 
- To achieve this, enclose the code that makes use of the lacking feature in an if statement that verifies the feature's support. 
- The polyfill code is run in its place if the feature is `not supported`.

---

### Example 1:

Let's imagine we wish to use the `Array.includes()` method. This method was added in `ECMAScript 2016`, but earlier browsers like `Internet Explorer 11` do not support it. We may build a polyfill in the manner described below to use this approach on earlier browsers:

---

![Array.includes](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/00i0l73evhfzpo3s14t2.png)

In this example, we use the! operator to see if `Array.prototype.includes()` is available. We implement the polyfill that offers a similar capability if it is not supported.

---

### Example 2:

Here's an example of implementing a polyfill for the `Object.assign()` method:

---

![Object.assign](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/qkfpiqn05l6n37xykn0y.png)

In this example, we first check whether `Object.assign()` is supported using the `typeof` operator. If it is not supported, we implement the polyfill that provides a similar functionality.

---

### Example 3:

Let's take an example of polyfill implementation for the `Math.trunc()` method. The function `Math.trunc(n)` "cuts off" the decimal portion of an integer. For example, `Math.trunc(1.23)` returns 1.

---

![Math.trunc](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/rdoo4uzzjp6x8ji7x549.png)

---


### Example 4:

Here's another example of a polyfill for the `Array.from()` method:

---

![Array.from](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/vt3d3cz9pbykko5gtmyl.png)

In this example, we use the `!` operator to first determine whether `Array.from()` is supported. We implement the polyfill that offers a similar capability if it is not supported.

---

### Example 5:

Here's another example of a polyfill for the `Promise` object:

---

```
// Check if Promise is supported
if (!window.Promise) {
    // Implement the polyfill
    window.Promise = function (executor) {
      if (typeof executor !== 'function') {
        throw new TypeError('Promise resolver ' + executor + ' is not a function');
      }
  
      var self = this;
      self.status = 'pending';
      self.value = undefined;
      self.reason = undefined;
      self.onFulfilledCallbacks = [];
      self.onRejectedCallbacks = [];
  
      function resolve(value) {
        if (self.status === 'pending') {
          self.status = 'fulfilled';
          self.value = value;
          self.onFulfilledCallbacks.forEach(function (callback) {
            callback(value);
          });
        }
      }
  
      function reject(reason) {
        if (self.status === 'pending') {
          self.status = 'rejected';
          self.reason = reason;
          self.onRejectedCallbacks.forEach(function (callback) {
            callback(reason);
          });
        }
      }
  
      try {
        executor(resolve, reject);
      } catch (e) {
        reject(e);
      }
    };
  
    window.Promise.prototype.then = function (onFulfilled, onRejected) {
      var self = this;
      var newPromise;
  
      if (self.status === 'fulfilled') {
        return newPromise = new Promise(function (resolve, reject) {
          try {
            var x = onFulfilled(self.value);
            resolve(x);
          } catch (e) {
            reject(e);
          }
        });
      } else if (self.status === 'rejected') {
        return newPromise = new Promise(function (resolve, reject) {
          try {
            var x = onRejected(self.reason);
            resolve(x);
          } catch (e) {
            reject(e);
          }
        });
      } else if (self.status === 'pending') {
        return newPromise = new Promise(function (resolve, reject) {
          self.onFulfilledCallbacks.push(function (value) {
            try {
              var x = onFulfilled(value);
              resolve(x);
            } catch (e) {
              reject(e);
            }
          });
  
          self.onRejectedCallbacks.push(function (reason) {
            try {
              var x = onRejected(reason);
              resolve(x);
            } catch (e) {
              reject(e);
            }
          });
        });
      }
    };
  
    window.Promise.prototype.catch = function (onRejected) {
      return this.then(null, onRejected);
    };
  }
```


In this example, we use the! operator to first determine whether the `Promise` object is supported. We implement the polyfill that offers a similar capability if it is not supported.

---

## Conclusion

Developers need `polyfills` because they enable them to use new capabilities in less modern browsers and contexts that don't support them. By utilising polyfills instead of writing special code for outdated browsers, developers can increase `browser compatibility`, improve `user experience`, increase `security` and save `time`. Using polyfills in your JavaScript code is a simple and straightforward process if you have the correct information and resources. Follow [Shubham Dutta](https://www.linkedin.com/in/shubham--dutta/) for more content like this.