How to Call a Nested Function from Another File in TypeScript

Learn how to effectively access a `nested function` from another file in TypeScript. Discover strategies for structuring your code to make it more modular and testable!
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Understanding Nested Functions in TypeScript

When working with TypeScript, you might find yourself needing to access a nested function from another file. But how does that work, especially when considering the block-scoping nature of let and const? In this guide, we will explore this situation and provide a clear solution.

The Problem: Accessing Nested Functions

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The Nature of Block-Scoped Functions

The reason you can't directly access foo2 is due to the block-scoping nature of let and const. When you declare a function (or variable) using these keywords within another function, they cannot be accessed outside of that function. They remain confined to their scope.

Key Points to Remember:

Block-scoped variables/functions using let and const are only accessible within their containing function, meaning you can’t call them from outside.

You can only call the inner function (like foo2) by invoking it within the outer function (like foo1).

Solution: Using an Object to Expose Nested Functions

To successfully access a nested function outside of its original file, consider declaring your functions within an object or class. This allows you to expose the desired functions or methods as properties. Here’s how you can do this:

Modify your code like this:

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Result Output

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Conclusion

By using objects or classes to encapsulate your functions, you not only make your code modular but also enhance its testability. This approach allows nested functions to be called from other files effectively.

Wrap-Up

In this guide, we covered:

The nature of nested functions and their accessibility

How to expose nested functionality using JavaScript objects

Hopefully, these insights into handling nested functions in TypeScript will make your development process smoother and more efficient. Happy coding!