The JavaScript Import Mistake Every Beginner Makes

You import a JavaScript file expecting to use a single function — and suddenly, a while loop starts running. If this has ever confused you, you’re not alone.

When learning JavaScript, importing files often feels simple at first — you expect to bring in a function and use it when needed. However, many beginners are surprised when code from an imported file runs automatically, even when they never explicitly call it.

This confusion usually shows up when:

• A while loop runs unexpectedly

• Console messages appear before your main code runs

• Output seems duplicated or out of order

• Terminal is showing output for functions which were never called in the current file (where user is trying to import )

  These issues are not bugs. They happen because of how JavaScript ES modules are designed to work. Any code written outside a function is called top-level code, and JavaScript executes it immediately when the module is imported

Understanding this behavior early is important because:

• It helps you write cleaner, predictable code

• It prevents unexpected execution

• It teaches proper module structure and separation of logic

In this blog, we’ll look at why imported JavaScript files execute automatically, what “top-level code” means, and how to structure your modules so that only the code you want runs.

Understanding JavaScript Imports

In JavaScript, when you write:

import { goToSleep, pet } from './Assignment02B_PetSimulator.js'

You might expect that only the exported function (goToSleep) will run.

But that’s not how JavaScript works.

The important rule here is, when a JavaScript file is imported, the entire file is executed from top to bottom.

This means:

• JavaScript does not import just the function

• It loads and runs the whole file

So any code written outside a function will run automatically.

For Example, in the current file where an import statement is written it, one might just expect output of the function “go to sleep()” .

But the output also has result of a “While” loop followed by results of the function “go to sleep()”.

If your imported file looks like this:

export let pet = { energy: 5 };

// This runs immediately on import
while (pet.energy < 10) {  
console.log("Pet is playing"); 
 pet.energy++;
}

export function goToSleep(pet) 
{  console.log("Pet is sleeping");}

Note that, the while loop is not inside any function, so it runs immediately when this file is imported.

That’s why you see output from the while loop — even if you never called it in your main file (the file where Import statement is specified)

Why This Feels Confusing

Many beginners think, “If I export a function, it should only run when I call it.” That idea is correct — but only for code inside functions.

JavaScript sees everything outside a function as instructions to run right away.

The Correct Way to Fix It

To prevent automatic execution, put your logic inside a function.

Fixed version:

export let pet = { energy: 5 };

//Adding while loop inside a function
function playWithPet() {
  while (pet.energy < 10) {  
  console.log("Pet is playing");   
 pet.energy++;  }
}

export function goToSleep(pet) {  console.log("Pet is sleeping");}

Now nothing runs automatically.

Using It in Another File

import { goToSleep, pet} from './Assignment02B_PetSimulator.js';

goToSleep(pet); // Only this runs

✔ The while loop runs only if you tag the associated function as export and then call playWithPet()

✔ Your program behaves exactly as expected

Examples in enterprise applications:

A utility file is created with all functions in it marked as “export”. Here no active calls to any function is made inside the file neither there are any loops outside of any function. And when this utility file is imported in some other .js file, it just executes the specified imported method(s) and not the entire utilities.js file.

Final Takeaway Points to Remember:

1. Code outside functions runs immediately when a file is imported.

2. Code inside functions runs only when called.

To summarize, importing a JavaScript file runs all top-level code. To control execution, always place logic inside functions.

Hope, you find this information useful. Happy Learning!