What is the process for configuring JavaScript polyfills to address ECMAScript compatibility across browsers?

The process for setting up JavaScript polyfills to deal with ECMAScript compatibility across various browsers can be quite a detailed task. This usually involves identifying the functionality that needs to be polyfilled, installing npm packages, importing the polyfills into your project, and configuring them correctly.

Here is a detailed process on how to configure JavaScript polyfills for ECMAScript compatibility.

Before you can implement polyfills, you must first figure out which ECMAScript features are supported in your target browsers and which are not. There are several resources available to help you with this, such as "Can I Use" and the Mozilla Developer Network (MDN). Identify the specific ECMAScript features that you're using in your code and cross-reference them with the compatibility tables to ensure that they're universally supported.

After you have identified the features you need to polyfill, you need to pick the right polyfills for your needs. Some of the most commonly used polyfills specific to ECMAScript features are core-js and polyfill.io.

For instance, if you want to use almost all ECMAScript features safe on any browser use core-js. You can install it using npm into your project:

npm install core-js

Besides, polyfill.io is a service which makes it easier to create script tags that automatically load the polyfills needed for the current browser. This means you don't need to worry about including each polyfill manually.

Once installed, you need to import the polyfills into your JavaScript code. You can do it at the entry point of your application. For instance, if core-js is used:

import "core-js";

This will include the entire core-js library. Alternatively, you can import just the specific features that you need:

import "core-js/es6/promise";
import "core-js/es6/array";

Babel is a widely used transpiler that can convert advanced ECMAScript code into a backward-compatible version that can be interpreted by older browsers.

When using Babel, we can make use of the babel-preset-env package, which enables us to only transpile the features that are not supported in the target browsers we define. babel-preset-env uses core-js under the hood for polyfilling such features.

First, you need to install Babel and babel-preset-env by running:

npm install --save-dev babel-core babel-preset-env

Then, you need to add a .babelrc file to your root directory. This file is used to configure Babel. For the presets part, add preset-env and in the target specify the browsers you want to target.

{
  "presets": [
    [
      "env",
      {
        "targets": {
          "browsers": [ ">0.25%", "not ie 11", "not op_mini all"]
        },
        "useBuiltIns": "usage", // will only include polyfills you used in your code
        "corejs": "3.6.4" // specify core-js version
      }
    ]
  ]
}

The "useBuiltIns": "usage" option configures babel-preset-env to automatically import only the polyfills that are used in your code.

By following these steps, you can configure JavaScript polyfills to address ECMAScript compatibility across browsers.