JavaScript - Runtime Environments

🕒 Created: 2024-12-22 | Updated: 2025-12-13

Node.js

node my_script.js

deno init
- A deno.json file is created to configure your project , and two TypeScript files are created: main.ts and main_test.ts .
  - The main.ts file is where you'll write your application code; initially, it will contain a simple program that adds two numbers together.
  - The main_test.ts file is where you can write tests; initially, it will contain a test for your addition program.

"You will now get all the benefits of Deno’s LSP, including IntelliSense, code formatting, linting, and more."
Setup:
- In the Extensions tab, search for "Deno" and install the extension by Denoland .
- Open the Command Palette by pressing Ctrl+Shift+P and type Deno: Initialize Workspace Configuration .
- That's it. A file called .vscode/settings.json will be created in your workspace.

Understanding the imports cache .
- deno info
  - Shows the cache directory of imports.
- deno --reload main.ts
  - Re-downloads the dependencies.
In Deno, the module system uses ESM (ECMAScript Modules), which is different from CommonJS (used in Node.js). Instead of module.exports and require , you use export and import .

deno main.ts

deno run main.ts

Deno will compile your TypeScript code to JavaScript with no extra config needed. Deno can also type check your TypeScript code without requiring a separate type checking tool like tsc .
Checks the file for static typing.

deno check module.ts

When using the deno run command, Deno will skip type-checking and run the code directly. To perform a type check of the module before execution occurs, you can use the --check flag with deno run .

deno run --check module.ts
# or also type check remote modules and npm packages
deno run --check=all module.ts

You can supply the --watch flag to deno run , deno test , deno compile , and deno fmt to enable the built-in file watcher.
The watcher enables automatic reloading of your application whenever changes are detected in the source files.