What are transpilers in javascript and why are they needed?

Anton Ioffe - September 11th 2023 - 14 minutes read

As experienced developers, we recognize that the world of JavaScript is continually evolving, introducing advanced syntax and language features that empower us to build increasingly innovative online experiences. Nevertheless, this continuous progression presents a recurring challenge: ensuring that our cutting-edge JavaScript code remains compatible with a wide array of runtime environments. This necessitates the use of powerful tools to keep our code running smoothly across different platforms, one of which is JavaScript transpilers.

In this insightful article, we'll be diving deep into the world of JavaScript transpilers, a critical aspect of modern web development often shrouded in mystery and misunderstanding. From introducing the basic concepts of transpiling in JavaScript to exploring its importance in ensuring compatibility, we'll peel back the layers of elective ignorance on this critical topic. We'll put tools like Babel and TypeScript under the spotlight, expounding on their functionality and showing you why they are a crucial part of any web developer's toolkit.

The discussion doesn't end there; we'll go the extra mile to deconstruct complex relationships between JavaScript transpilers and compilers and contrast the differences between Babel and TypeScript. For the cherry on top, prepare to delve into advanced concepts like polyfilling, the usage of ECMAScript 2015 (ES6), and best practices for transpiling in modern web development. Get ready to enhance your web development prowess and navigate the ever-evolving JavaScript landscape with confidence.

Understanding Transpilers: Definition and Basics in JavaScript Programming.

Understanding transpilers begins with the basic definition. A transpiler is a type of compiler that takes the source code of a program written in one programming language and translates it into an equivalent and compatible program in another language. With JavaScript programming, transpilers translate modern JavaScript (ES6 and beyond) into backwards-compatible versions of JavaScript that can be run in older environments that may not support newer language features. This transpiling process allows developers to write code using modern syntactic sugar and features while ensuring the broadest possible compatibility.

To illustrate how a transpiler works, let's look at a basic code snippet:

let greeting = 'Hello, Transpilers!'

The code above is written in ES6 JavaScript, which uses the let keyword for variable declaration. However, older JavaScript engines may not understand the let keyword. Therefore, a transpiler would translate this JavaScript into something more universally understood, such as:

var greeting = 'Hello, Transpilers!'

This "transpiled" version of our original JavaScript code uses the var keyword, making it readable to older JavaScript engines.

One of the common mistakes developers make when writing JavaScript for transpilation is to assume the translated code will have the same performance characteristics as the original code.

Here's an example:

Initial example using the spread operator in ES6:

let arr1 = [1, 2, 3];
let arr2 = [...arr1];  

Transpiled version to ES5 syntax:

var arr1 = [1, 2, 3];
var arr2 = Array.prototype.slice.call(arr1);

While it would be convenient to assume that these two code blocks will perform similarly, this is not the case. ES6 spread syntax is considerably faster than using the Array.prototype.slice.call method. Consequently, if performance is critical in your application, it's essential to choose newer ES features carefully and understand the implications of transpiled code.

Transpilers play a vital role in JavaScript programming, allowing the use of next-generation JavaScript, safe in the knowledge that the deployed code will be suited for the wide variety of JavaScript engines in the wild. The understanding of how transpilers function and their implication for deployed code is an essential part of modern JavaScript development.

The Need for Transpiling: Compatibility and Modern Web Development.

Transpiling, short for 'transforming' and 'compiling', is a process where source code written in one language is converted into another language that has a similar level of abstraction. It has become a practical necessity in modern web development, particularly when working with JavaScript. As a language, JavaScript has been evolving at a startling pace. However, not all runtime environments (also known as the client's web browsers) can keep up with these rapid updates and changes. This discrepancy can lead to compatibility issues, and this is where transpiling plays a pivotal role.

Why JavaScript Transpiling?

In web development, compatibility is not just an afterthought; it's a priority. The issue here is not about developers adopting changes faster than browsers, but rather about ensuring that these changes are effectively accepted by the different technologies supporting web browsers. It can be frustrating to spend hours debugging only to find out that a specific JavaScript feature doesn't work on a certain browser. Ensuring compatibility across various runtime environments is therefore paramount to delivering a positive user experience.

Transpilers play a crucial role in bridging this gap. They allow developers to use ES6+ JavaScript syntax and then transpile it back into ES5 – a JavaScript version most browsers can interpret.

An excellent example is Arrow functions usage, which was introduced in ES6 to provide a shorthand syntax for defining function expressions. Let's illustrate this with a code example:

// Initial code with arrow functions
const greet = () => {
    console.log('Hello World');

Upon transpiling the above ES6 code into ES5, it transforms into:

// Transpiled ES5 equivalent
var greet = function() {
    console.log('Hello World');

It's a common oversight to assume that all browsers interpret JavaScript in the same way. Can you recall a time when you assumed this and ran into cross-browser compatibility issues?

Furthermore, ES6 introduced a module system with import and export keywords. Transpiling assists developers in leveraging this ES6 module system, breaking down code into self-contained pieces. This enhances code management, improving modularity and reusability. Here is a snippet illustrating how import and export get utilized in ES6:

// myModule.js
export const greet = 'Hello world!';

// App.js
import { greet } from './myModule.js';
console.log( greet );

Once transpiled back to ES5, the code transforms into:

// myModule.js
exports.greet = 'Hello world!';

// App.js
var greet = require('./myModule.js').greet;
console.log( greet );

Transpiling Drawbacks and Performance Implications

Despite being instrumental in modern web development, transpiling does have drawbacks, mainly the potential for code inflation, which results in larger file sizes and consequently, slower web page load times.

Debugging can also prove difficult. For instance, imagine finding a bug in your code, only to view the transpiled code in the browser and struggling to correlate it with your original logic. Developers often resort to source maps to map the transpiled code back to their original code, thus simplifying debugging.

However, despite these difficulties, most developers find that the benefits of transpiling far outweigh any negatives. Transpiling enables them to utilize current language features while ensuring backward compatibility, a crucial factor in the fast-paced evolution of web development.

So, when thinking about your own experiences with JavaScript, have you ever had to transpile code? Do you think using a transpiler has streamlined your development process, or has it introduced unforeseen challenges? If so, how have you tackled these issues?

A Common Transpiling Mistake

A common oversight made by developers new to transpiling is ignoring potential issues around browser-specific interpretations of JavaScript features. Failing to transpile your code can result in features not working correctly in all browsers, which can be difficult to debug.

For instance, you might find that using a relatively new array method like .includes() works perfectly in Chrome but breaks your site in an older version of Internet Explorer.

// Example code using the .includes() array method
const array = [1, 2, 3];

if(array.includes(2)) {
    console.log('Number 2 is in the array');

In this case, the correct approach would be to transpile the ES6 code to ES5:

// Correct ES5 equivalent
var array = [1, 2, 3];

if(array.indexOf(2) !== -1) {
    console.log('Number 2 is in the array');

So it's always a good idea to use a transpiler to ensure all your code will run as expected, regardless of the browser.

The Role and Functionality of Babel in JavaScript Transpilation.

Babel is one of the most widely used JavaScript transpilers today. It functions as a bridge between the latest JavaScript features and the JavaScript features currently supported by various browsers. This is essential because not all browsers immediately support all the newest JavaScript features, and programmers must ensure compatibility for their users while still having the ability to exploit the latest features.

Babel Functionality

Babel works by parsing your JavaScript code into an Abstract Syntax Tree (AST), performing transformations on this AST, and then regenerating JavaScript code from the transformed AST. This transpiled JavaScript is more likely to be compatible with older browsers.

To transpile a simple JavaScript function with Babel, you would use the transform method, as illustrated below:

const babel = require('@babel/core');
// Comment comes before the code line
const code = 'function Greet(name) { return `Hello, ${name}!`; }';

const result = babel.transform(code, { presets: ['@babel/preset-env'] });

This code block introduces the @babel/preset-env preset, which enables transformations for ES2015+.

Babel is not just nailed down to transpiling newer JavaScript syntax to older syntax. It's also capable of JSX syntax and Flow, which are not standard JavaScript but have found widespread acceptance.

Common Mistakes while Using Babel

One common misstep involves attempts to use newer JavaScript features that require more than just syntax transformations. Babel can convert newer syntax to older syntax, but when it comes to polyfilling new JavaScript APIs, it falls short. The typical workaround for this is to use @babel/polyfill.

This code segment uses the Array.includes method, which isn't supported in older browsers.

import '@babel/polyfill';
// The Array includes method is not available in some older browsers
const arr = [1, 2, 3];
const doesInclude = arr.includes(2);

If you transpile this with Babel, it won't work in those older browsers because they lack the Array.includes method. For compatibility's sake, @babel/polyfill should be included in your project as demonstrated in the code block above.

Another common pitfall is the incorrect setting up of Babel. It's crucial to comprehend the functions of each Babel preset and plugin and to identity ones that align with your project requirements. Misconfigurations often lead to unusual bugs and incorrect transpilation output.

In conclusion, Babel functions as an indispensable tool in a JavaScript developer's toolbox. When used and configured accurately, it lets developers use innovative JavaScript features whilst ensuring browser compatibility. Babel allows developers to write modern, clean, and consistent JavaScript and support a broad range of browsers. In the following sections, we'll delve deeper into Babel, explaining various plugins, presets, and how to fine-tune them for your specific objectives.

Take a moment to reflect. Have you been making the most of Babel? Are there parts of your project where Babel's more thorough application could enhance compatibility and readability? Are there legacy codebases within your realm of influence that could benefit immensely from Babel's judicious use? Consider these questions as part of your journey towards optimizing Babel's utilization.

Differences and Relationships: Babel vs TypeScript and Transpiler vs. Compiler.

When working with JavaScript, you'll come across two vital tools: Babel and TypeScript. These tools are renowned in the world of transpiling and compiling, respectively. What are the differences between them, and which tool should you choose? In the complex realm called JavaScript, does it matter if you opt to transpile or compile? Dive deep into these relationships and unravel the intricacies that lie beneath with real-code examples and common mistakes of our developer lives.

Babel vs. TypeScript

Babel and TypeScript are two popular technologies used in modern web development, however, they have notable differences concerning purpose, functionality, and ideal use-cases.

Babel is primarily a transpiler. It's been designed to convert new JavaScript ES6+ code into backwards compatible versions for old browsers that may not support the latest features. Let's take a look at a simple example using arrow functions, which is an ES6+ feature:

const arrowFunction = () => {
    console.log('Hello from Babel!');

When transpiled with Babel, we get:

var arrowFunction = function arrowFunction() {
    console.log('Hello from Babel!');

The ES6 arrow functions get turned into ES5 equivalent function expressions, which is more compatible across older browsers.

On the other hand, TypeScript is a typed superset of JavaScript that adds static types to the language. It's a compiled language where TypeScript code is converted or compiled into JavaScript. TypeScript’s main aim is to catch errors during the development phase rather than at runtime.

function greeter(person: string) {
    return 'Hello, ' + person;

let userName = 'TypeScript user';

This will compile into regular JavaScript:

function greeter(person) {
    return 'Hello, ' + person;

var userName = 'TypeScript user';

Note that if we try passing a number to the greeter function in TypeScript, it will throw a compile-time error because greeter expects a string and not a number.

Transpiler vs. Compiler

Next up, understanding the concept of a transpiler and a compiler is equally crucial. While the terms often seem interchangeable, there lies a minute yet significant difference.

A transpiler, or source-to-source compiler, transforms code between similar programming languages at the same abstraction level. For instance, ES6 to ES5 JavaScript, as illustrated in the Babel example. Transpiler output is meant to be read and edited by developers, hence readability is emphasized.

Meanwhile, a compiler transforms code from one programming language level to a lower one. For instance, TypeScript to JavaScript, or C/C++ to Assembly Language, or to Machine code. In compiled languages, the code that you write isn't directly executed by the machine. It has to be translated first to the language that machines understand.

Common mistake: Confusing between transpiling and compiling can lead to difficulties when choosing the appropriate tool – Babel can handle newer feature transpilation but doesn’t have type checking, while TypeScript checks types but doesn’t tackle all new JS features.

Using a transpiler and a compiler together can create a robust codebase with fewer bugs and back-compatibility. For example, using TypeScript for type-checking and then Babel for transpiling the TypeScript code into browser-compatible JavaScript covers the advantages of both.

What sort of projects have you worked on? At what stage did you feel the requirement for transpiling or type-checking, or both?

Remember, making informed decisions about your build and transpiling processes based on the trade-offs among performance, memory, complexity, readability, modularity, and reusability can be crucial in ensuring smooth development and improvement of your web projects. Utilize these transformations strategically to elevate the quality of your JavaScript programming.

Advanced Concepts: Polyfilling, ES6, and Transpiling Best Practices in Web Development.

Polyfilling in JavaScript

Polyfilling is an unavoidable aspect of JavaScript development. It refers to the writing of code that incorporates a feature into web browsers that were initially not designed to support that feature. In essence, polyfills serve as bridges, bridging the gap between modern and older browsers by providing missing functionalities.

A common polyfill is for the Array.prototype.includes method. Below is a typical implementation of this polyfill:

if (!Array.prototype.includes) {
  Array.prototype.includes = function(searchElement, fromIndex) {
    if (this == null) {
      throw new TypeError('"this" is null or not defined');

    var o = Object(this);
    var len = o.length >>> 0;

    if (len === 0) {
      return false;

    var n = fromIndex | 0;
    var k = Math.max(n >= 0 ? n : len - Math.abs(n), 0);

    function sameZero(x, y) {
      return x === y || (typeof x === 'number' && typeof y === 'number' && isNaN(x) && isNaN(y));

    while (k < len) {
      if (sameZero(o[k], searchElement)) {
        return true;

    return false;

While polyfills are beneficial, a commonly made mistake is to indiscriminately add them without verifying their need. The unnecessary proliferation of polyfills can lead to bloated code, a decrease in performance, and conflict with existing or future features. By writing reusable polyfills, not only is time saved, but it also contributes to maintaining clean and efficient code.

ES6 and Transpiling

With the introduction of ECMAScript 2015, also known as ES6, the complexity of JavaScript backward compatibility was intensified. The array of new features introduced by ES6 led developers to grapple with the realization that 'the modern' does not necessarily equate to 'universally supported.'

That's when a transpiler becomes invaluable, as it converts ES6 features like arrow functions to ES5 to ensure compatibility with some browsers that are not yet supportive of these changes. Let's observe the example below:

// ES6 code
const square = num => num * num;

// Transpiled ES5 code
var square = function(num) {
  return num * num;

In the ES6 function, we find arrow notation and the const keyword, which would not be understood by an ES5-based browser. The transpiler converts it to a function declaration and replaces the const keyword with var, making it compatible with earlier versions.

A mistake often made is assuming that all modern browsers support ES6, neglecting the crucial act of transpiling. Although modern browsers offer robust support for ES6, it's crucial to remember the diverse user base interacting with your site, which includes older and less popular browsers. Further, the mismanagement of complex memory and computational details can lead to inefficiencies in managing resources.

Transpiling Best Practices

Adopting transpiling requires adhering to certain best practices:

  • Transpile Only What You Need: Avoid transpiling your entire codebase automatically. Conduct a dependency analysis to identify the features that are incompatible with your target browsers. This approach avoids inadvertently bloating your modules with unnecessary transpilation, and hinders the site's smooth operation.
  • Use Strict Mode: Always use strict mode ('use strict') when writing JavaScript. This practice can help prevent subtle issues that modern browsers may overlook, but which might become major issues in older browsers after transpiling.
  • Test on Multiple Platforms and Browsers: Make sure to test your transpiled code on various browsers and devices to avoid assumptions about its performance.

Contemplations for the Reader

  1. Can you envision scenarios where transpiling may not be the ideal solution?
  2. What are the potential impacts of indiscriminate use of polyfills or uncalculated transpiling on complexity and memory?
  3. Have you encountered issues that arose from miscalculations regarding the need for transpiling?


The article tackles the intricate topic of transpilation, focusing on JavaScript, its importance, applications, and potential pitfalls in modern web development. It underscores the critical role of transpilers like Babel and TypeScript in enabling developers to leverage advanced JavaScript syntax and features, while ensuring compatibility with various runtime environments. The article further delves into key concepts such as polyfilling, transpiling best practices, and the interwoven relationship between JavaScript transpilers and compilers.

The piece further provides nuanced insights into the globally-acclaimed tools, Babel and TypeScript, explicating their functionalities, critical distinctions, and appropriate application in specific use-cases to help users make informed decisions based on differing contexts. By providing concrete code examples and common mistakes, the reader is equipped with a comprehensive understanding of the intricacies of JavaScript transpilation and the impact of varying practices on performance, complexity, and memory.

To deepen understanding, readers are encouraged to apply the knowledge by conducting an audit of a current JavaScript project. The task is to identify code snippets that use ES6+ syntax, and then use Babel to transpile these snippets to ES5 script, ensuring compatibility across a range of browsers. This dynamic exercise will enlighten and challenge readers on the practical aspects of JavaScript transpiling, enabling them to gain real-world, hands-on exposure, and thereby solidify their conceptual understanding.

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