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.
Different engines have different “codenames”. For example:
- V8 – in Chrome, Opera and Edge.
- SpiderMonkey – in Firefox.
The terms above are good to remember because they are used in developer articles on the internet. We’ll use them too. For instance, if “a feature X is supported by V8”, then it probably works in Chrome, Opera and Edge.
Engines are complicated. But the basics are easy.
- The engine (embedded if it’s a browser) reads (“parses”) the script.
- Then it converts (“compiles”) the script to machine code.
- And then the machine code runs, pretty fast.
The engine applies optimizations at each step of the process. It even watches the compiled script as it runs, analyzes the data that flows through it, and further optimizes the machine code based on that knowledge.
- Add new HTML to the page, change the existing content, modify styles.
- React to user actions, run on mouse clicks, pointer movements, key presses.
- Send requests over the network to remote servers, download and upload files (so-called AJAX and COMET technologies).
- Get and set cookies, ask questions to the visitor, show messages.
- Remember the data on the client-side (“local storage”).
Examples of such restrictions include:
Modern browsers allow it to work with files, but the access is limited and only provided if the user does certain actions, like “dropping” a file into a browser window or selecting it via an
This limitation is, again, for the user’s safety. A page from
http://anysite.comwhich a user has opened must not be able to access another browser tab with the URL
http://gmail.com, for example, and steal information from there.
- Full integration with HTML/CSS.
- Simple things are done simply.
- Supported by all major browsers and enabled by default.
That’s to be expected, because projects and requirements are different for everyone.
Modern tools make the transpilation very fast and transparent, actually allowing developers to code in another language and auto-converting it “under the hood”.
Examples of such languages:
- TypeScript is concentrated on adding “strict data typing” to simplify the development and support of complex systems. It is developed by Microsoft.
- Flow also adds data typing, but in a different way. Developed by Facebook.
- Kotlin is a modern, concise and safe programming language that can target the browser or Node.