Let’s see what’s so special about JavaScript, what we can achieve with it, and what other technologies play well with it.
JavaScript was initially created to “make web pages alive.”
The programs in this language are called scripts. They can be written right in a web page’s HTML and run automatically as the page loads.
Scripts are provided and executed as plain text. They don’t need special preparation or compilation to run.
In this aspect, JavaScript is very different from another language called Java.
When JavaScript was created, it initially had another name: “LiveScript.” But Java was very popular at that time, so it was decided that positioning a new language as a “younger brother” of Java would help.
However, JavaScript became a fully independent language with its own specification called ECMAScript, and now it has no relation to Java at all.
Today, JavaScript can execute:
- In the browser
- On the server
- On any device with a JavaScript engine
Different browsers use different JavaScript engines:
V8– in Chrome, Opera, and EdgeSpiderMonkey– in Firefox- Others:
Chakra(IE),JavaScriptCore,Nitro,SquirrelFish(Safari)
These terms are useful to know because they’re often mentioned in developer articles.
JavaScript engines are complex, but here’s a simplified explanation:
- The engine parses the script.
- It compiles it into machine code.
- The machine code is executed.
The engine also applies optimizations at every step. It even monitors how the code runs and dynamically improves performance as it executes.
Modern in-browser JavaScript is a safe language. It doesn't give access to:
- Memory
- The CPU
Instead, it can:
- Add or change HTML and CSS on the page
- Respond to user interactions (clicks, key presses)
- Send/receive requests (AJAX/COMET)
- Work with cookies
- Show alerts, confirmations, prompts
- Store data locally (
localStorage,sessionStorage)
JavaScript in the browser has limitations for security reasons:
- Cannot read/write arbitrary files or access OS-level functions
- File access is limited to user-initiated actions (e.g. selecting files)
- Cannot access cameras/microphones without explicit permission
- Cannot freely interact with other tabs/windows (due to Same Origin Policy)
- Cannot send/receive data from another domain unless explicitly allowed (via CORS)
Outside of the browser (e.g. in Node.js), many of these limitations do not apply.
JavaScript stands out because of:
- Full integration with HTML/CSS
- Simple things are simple to do
- Supported by all major browsers by default
JavaScript is the only technology that brings these together, making it the most widespread tool for creating web interfaces.
It can also be used to:
- Build servers
- Create mobile apps
- Develop desktop applications
Many developers want features not natively available in JavaScript. So several transpiled languages have emerged:
| Language | Highlights | Developed By |
|---|---|---|
| CoffeeScript | Cleaner, shorter syntax (loved by Ruby devs) | — |
| TypeScript | Strong typing for large-scale systems | Microsoft |
| Flow | Static typing alternative | |
| Dart | Can run in its own engine or transpile to JS | |
| Brython | Allows writing browser code in Python | — |
| Kotlin | Concise, safe, targets browser or Node | JetBrains |
Even if you use one of these, knowing JavaScript is essential.
- JavaScript began as a browser-only language, but now runs in many environments.
- It is the most widely-adopted browser scripting language.
- There are many transpiled languages, but understanding JavaScript remains critical for web development.