Polyfills in React Application: A Complete Guide
Tags: javascript, polyfills, webdev, react
Published: 2023-04-17 20:54:03

## Overview 

A well-liked JavaScript library for creating user interfaces is called React. Developers can easily and effectively manage state with React and build reusable components. But when several browsers and gadgets don't support the same functionalities, there may be compatibility problems, just as with any JavaScript programme. Polyfills are useful in this situation.

---

## Describe Polyfills

Code fragments called `polyfills` offer functionality for features that might not be supported by all `browsers` or `operating systems`. They enable programmers to guarantee that their code operates as intended across all platforms, regardless of any restrictions on feature support.

If you want to know more about `polyfills` go to this [link](https://dev.to/shubhamdutta2000/polyfills-for-javascript-a-full-overview-3f7m)

---


## Why Do React Users Use Polyfills?

Making sure a React application is compatible with as many browsers as feasible is crucial when developing one. This is crucial for developing online applications that consumers may access on a variety of platforms and devices.

Many of the contemporary JavaScript features used by React might not be supported by earlier browsers like Internet Explorer. By providing fallback code for features that aren't supported, polyfills can aid in ensuring that these features function properly across all browsers.

---

## Examples of Polyfills in React

Here are some more examples of popular JavaScript features used in React that may require polyfills:

1. **Promise**:
  - Promises are used in React for asynchronous operations. However, they are not supported in older versions of Internet Explorer. 
  - To use Promises in all browsers, you can add a polyfill for them, such as the es6-promise package. 
  - This package provides a Promise implementation that works in older browsers, allowing you to use Promises in your React application without any compatibility issues.


2. **Object.assign()**: 
  - This method is used in React for merging objects. However, it is not supported in older versions of Internet Explorer. 
  - To use Object.assign() in all browsers, you can add a polyfill for it, such as the object-assign package. 
  - This package provides an Object.assign() implementation that works in older browsers, allowing you to use this method in your React application without any compatibility issues.


3. **Fetch API**: 
  - The `Fetch API` is used in React for making HTTP requests. However, it is not supported in older versions of `Internet Explorer`. 
  - To use the Fetch API in all browsers, you can add a polyfill for it, such as the `whatwg-fetch` package. 
  - This package provides a Fetch API implementation that works in `older browsers`, allowing you to use this feature in your React application without any compatibility issues.


4. **Symbol**: 
  - `Symbols` are used in React for creating unique identifiers. However, they are not supported in some older browsers. 
  - To use `Symbols` in your React application, you can add a polyfill for them, such as the `core-js `package. 
  - This package provides a `Symbol` implementation that works in `older browsers`, allowing you to use this feature in your React application without any compatibility issues.


5. **String.startsWith()**:
  - `startsWith` method determines whether a string begins with the characters of a specified string, returning true or false as appropriate. However, it is not supported in older versions of `Internet Explorer`.
  - To use `startsWith` in your React application, you can add a polyfill for them, such as the core-js package.
  - This package provides a `String.prototype.startsWith()` implementation that works in `older browsers`, allowing you to use this method in your React application without any compatibility issues.


6. **Array.includes()**:
  - The `Array.includes()` method in React is used to determine whether an array contains a specific value. Older versions of `Internet Explorer` do not, however, support it. 
  - You can include a `polyfill` for `Array.includes()`, such as the `array-includes` package, to make it work in all browsers. 
  - You can use the `Array.includes()` method in your `React` application without encountering any compatibility problems thanks to this package's implementation, which supports older browsers.


7. **Object.entries()**: 
  - This method is used in React for getting an object's key-value pairs as an array. However, it is not supported in older versions of `Internet Explorer`. 
  - To use `Object.entries()` in all browsers, you can add a polyfill for it, such as the `object.entries` package. 
  - This package provides an `Object.entries()` implementation that works in `older browsers`, allowing you to use this method in your React application without any compatibility issues.

---


## Writing polyfills from scratch

Here is an example of how to write a polyfill from scratch for the `startsWith` method in React:

![Polyfills from scratch 1](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/488oh1n8xc4nkf7p5nij.png)

In this example, we first determine whether the String prototype's `startsWith` method already exists. If it doesn't already exist, we define it as a function that accepts `search` and `pos` as parameters.

The `substr` method is then used by the function to extract a section of the string beginning with the `pos` argument, or 0 in the absence of `pos` or if `pos` is less than 0. The search parameter is then compared to this substring using rigorous equality (===), and a `boolean` result is returned.

The `startsWith` method may be made to function as intended in all browsers, even those that don't support it natively, by specifying this polyfill.

---

Here's another example of how to create a polyfill for the `array includes` method:


![Polyfills from scratch 2](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/1xo1s3t22xz1359kd7h4.png)

This polyfill verifies whether the `Array.prototype` object already defines the `includes` method. It generates a new function that accepts an `elementToBeSearched` parameter if it is not defined.

The function then used a for loop to iterate through the array, inspecting each element to see if it matches the `elementToBeSearched` parameter. It returns `true` if it discovers a match. The function returns `false` if it cannot locate a match.

If you want to write more `polyfill` from scratch go to this [link](https://dev.to/shubhamdutta2000/polyfills-for-javascript-a-full-overview-3f7m)

### NOTE:

It's important to note that writing a `polyfill` from `scratch` can be challenging, especially for `complex features`. In addition, it's important to thoroughly test the polyfill in various `browsers` and `environments` to ensure it behaves correctly and doesn't cause any unexpected issues.


---

## How to Add Polyfills in React

You can use a library containing a collection of polyfills that will perform the same function for you instead of manually checking for the existence of a feature and adding a polyfill.

Use pre-existing polyfills or packages whenever possible; only create your own if absolutely essential and if you have the necessary knowledge and experience.

--

### Example 1:

Adding polyfills in React is a simple process. Here's how you can add a polyfill for `startsWith`:


- Install the core-js package as a dependency in your project:

  ```
  npm install core-js
  ```

- Import the startsWith polyfill at the top of your JavaScript file:

  ```
  import 'core-js/actual/string/starts-with';
  ```


- Now you can use the startsWith method in your React components, like this:

  ```
  const testString = 'Hi all JavaScript Developer out there!';
  const startsWithHi = testString.startsWith('Hi');

  console.log(startsWithHi); // true
  ```

We can make sure that the `startsWith` method functions as intended in all browsers, even those that don't support it natively, by importing the `startsWith` polyfill from `core-js`.

---

### Example 2:

Let's take another example of how to add a polyfill for Array.includes():

- Install the array-includes package using npm:

  ```
  npm install array-includes
  ```


- Import the package in your JavaScript code:

  ```
  import 'array-includes/auto';
  ```

This will automatically add the `polyfill` for `Array.includes()` to your React application.

---

### Example 3:

Here is another example of how to use a polyfill for `Promise` in a React project:


- Install the `es6-promise` package using `npm`:

  ```
  npm install es6-promise
  ```


- Import the package in your JavaScript code:

  ```
  import 'es6-promise/auto';
  ```

This will automatically add the polyfill for Promise to your React application.

---

## Conclusion

Polyfills are an essential tool for ensuring browser compatibility in React applications. By providing fallback code for unsupported features, polyfills help developers create web applications that work seamlessly across all browsers and platforms. 

If you're developing a React application, it's important to consider the use of polyfills to ensure maximum compatibility. With the right polyfills in place, you can be confident that your application will work as intended, regardless of any feature support limitations in the browser or environment.

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