How to define global variable in Deno?

Defining global variables in Deno differs from other JavaScript environments because Deno operates in a more secure runtime by restricting certain global operations to enhance security.

Method One: Using the Global Object

In Deno, you can define global variables using the global object globalThis. This is the simplest and most direct approach. For example:

javascript
// Set global variable
globalThis.myGlobalVar = 'Hello, Deno!';

// Use the global variable elsewhere
console.log(globalThis.myGlobalVar); // Output: Hello, Deno!

Method Two: Module System

Although not traditional global variables, you can store and export variables in a dedicated module and import them where needed. This method aligns better with Deno's modular design philosophy and effectively manages dependencies and variable scope.

Create a config.ts file:

typescript
// config.ts
export const API_KEY = 'YOUR_API_KEY_HERE';

Import and use it in other modules:

typescript
// main.ts
import { API_KEY } from './config.ts';

console.log(API_KEY); // Use the imported variable

Usage Scenario Example

Suppose you are developing a Deno application that needs to share a database connection string across multiple modules. Using globalThis might be a quick solution, but using the module system (the second method) is safer and more maintainable. You can create a databaseConfig.ts module, define the connection string within it, and import it in files where database operations are performed.

This approach ensures all configurations are managed in one centralized location, facilitating future maintenance and modifications.

Summary

Although global variables can be defined in Deno using globalThis, it is recommended to use the module system for handling data shared across modules. This approach is safer and better aligns with Deno's design principles.