Lesson · JavaScript Basics
Run your first JavaScript
JavaScript can run in more than one environment, so knowing where a snippet runs matters.
What you will learn
- Run a Node.js script
- Recognize browser-only APIs
- Read console output
Choose an environment
Node.js runs JavaScript outside a browser and is useful for small terminal programs. A browser runs JavaScript alongside a web page and provides APIs such as document. Those environments share the language but not every built-in feature, so an example should say which environment it expects.
Save and run a file
Save source code in a plain text file with a .js extension. With Node.js installed, run node hello.js from the folder containing the file. The terminal command is separate from the JavaScript inside the file, just as a Python command starts its own interpreter.
Use the console as evidence
console.log writes a value to the environment's developer console or terminal output. It is useful while learning, but it is not a page heading or an alert shown to every visitor. Read exactly what appears, then compare it with the value the code was intended to produce.
Keep examples portable
A snippet using console.log and basic values can run in both Node and a browser console. Code that refers to document needs a browser page. If Node reports that document is undefined, the code likely expects a browser rather than a different JavaScript syntax.
Load browser scripts at the right time
When JavaScript needs page elements, use a script element with defer or place the script after the relevant markup so the document has been parsed first. A selector can return null when the requested element is absent. Check that possibility when code supports multiple pages instead of assuming every page contains the same controls.
console.log('Hello, JavaScript!');Expected output
Hello, JavaScript!Practice exercise
Create a JavaScript file that logs a greeting and the name of a topic you want to learn.
Show a hint
Use two console.log calls or one template literal.
Show one possible solution
For example, console.log('Hello!'); console.log('Today I will learn arrays.'); prints the two messages on separate lines.
Quick check
document is a browser API representing the page; Node does not provide a page by default.