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Date: 2024-12-30 15:29:54
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Tern Systems, here is a thoughtful answer for that Stack Overflow question:


Short Answer
In modern JavaScript modules (<script type="module">), variables are scoped to the module itself and are not automatically attached to the global object (window). Consequently, code in a non-module script cannot directly access variables imported or declared in a module script. You need to explicitly export them from your module or attach them to the global object if you want to share them with non-module scripts.


Detailed Explanation

  1. Module Scope Isolation
    When you use <script type="module">, any variables, classes, or functions declared within that script are module-scoped. They do not leak into the global scope the way variables in normal <script> blocks do. This design prevents unexpected collisions and promotes better encapsulation.

  2. Why You Can’t Access Module Variables in Non-Modules

    • Non-module scripts (or the console if you’re testing directly from a browser DevTools console) look for variables on the window object, which by default only contains variables declared in global scope.
    • If your script is a module, anything declared inside remains within the module’s local scope unless explicitly exported.
  3. How to Expose Variables from a Module
    If you want non-module scripts (or the global scope) to see your module’s variables:

    • Attach to the window object
      // Inside your module:
      const myValue = 42;
      window.myValue = myValue;
      
      Now a non-module script (or even inline code in the HTML) can do:
      console.log(window.myValue); // 42
      
    • Import/Export Mechanism (for module-to-module communication)
      If both scripts are modules, you could export from one module and import in another:
      // moduleA.js
      export const myValue = 42;
      
      <!-- moduleB -->
      <script type="module">
        import { myValue } from './moduleA.js';
        console.log(myValue); // 42
      </script>
      
  4. Ensure Consistent Script Types

    • If you’re mixing module scripts and non-module scripts, be aware that variables declared in one scope aren’t automatically available in the other.
    • For certain projects, it’s simpler to keep all scripts as modules for consistent import/export patterns.

Practical Recommendations

  1. Convert Dependent Scripts to Modules
    Whenever possible, convert all <script> tags to type="module">. This way, you can leverage JavaScript’s native module system, using export and import as intended.

  2. Use the Global Object Only When Necessary
    If you genuinely need global access—like for a quick proof of concept—attaching to window is a simple approach. However, from a design standpoint, global variables can lead to naming collisions and maintenance issues. Use them judiciously.

  3. Check Browser Support
    Most modern browsers fully support <script type="module">, but you might need to transpile or use polyfills for older environments. Keep that in mind if you’re targeting legacy browsers.


Module scripts are designed to provide a more robust and encapsulated code structure. If you need to share variables with non-module scripts, explicitly assign them to the global scope or refactor your code to use only modules. By following the import/export standards, you’ll maintain cleaner, more maintainable code.

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Posted by: Tern