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Discover how to manage `thenable` functions in JavaScript/TypeScript that behave differently when called directly or when `await`/`then` is used.
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Understanding Thenable Functions: Returning Different Types in JavaScript and TypeScript

In the world of JavaScript and TypeScript, thenable functions often create confusion, particularly when they need to return different types of values based on how they are called. This guide will dive into the concept of thenable functions, exploring how they can behave like functions in one scenario and like promises in another. We'll clarify how to achieve this behavior and discuss potential solutions to implement it in your code.

The Problem at Hand

Let's start by defining what a thenable function is. A thenable is an object or function that has a then method. This allows it to be used in promise-like ways. However, the challenge arises when you want a thenable function to return different types depending on how it is invoked.

For instance:

When called like a function, it returns an array of strings: ['just function']

When awaited or given a then, it returns a single string: 'like a Promise'

Here’s a simple example in JavaScript:

[[See Video to Reveal this Text or Code Snippet]]

The main question is: Is it possible to type this behavior using TypeScript?

The Answer: No, It’s Not Possible

Unfortunately, the straightforward answer is no. A function cannot inherently determine whether it is being called directly or being treated as a thenable because the call context is ambiguous. The function doesn't know whether to provide an array or a string without additional context or structure around it.

Alternative Approaches

While directly implementing such functionality isn't feasible, there are some workarounds you could consider. However, keep in mind that they can lead to added complexity in your code. Let's explore some of these methods:

Returning a Function
One approach is to return a function that itself returns a value. This nested function can behave differently when then is applied.

[[See Video to Reveal this Text or Code Snippet]]

Returning an Object with a then Method
Another alternative is to return an object that has a then method along with other methods like toString or valueOf. This way, you can define different behaviors based on how the object is used.

[[See Video to Reveal this Text or Code Snippet]]

Using a Thenable Object with Additional Methods
A third option would be to define a thenable object that has a regular method, such as now, to retrieve the desired value when not using promises.

[[See Video to Reveal this Text or Code Snippet]]

Which Approach to Choose?

Among these alternatives, the third approach stands out as the clearest and easiest to maintain if you need to manage different return types. However, if you can avoid this complexity, using two separate functions—each with defined behavior—might likely be the most straightforward solution.

Conclusion

In conclusion, while it's not directly possible to create a single thenable function that behaves differently based on how it is invoked, there are various strategies you can employ to achieve something similar. Choosing the right approach will depend on the specific needs of your application, and careful consideration is recommended to avoid maintenance issues down the line.

By understanding the limitations of thenable functions and exploring creative solutions, you will become more adept at leveraging the full power of JavaScript and TypeScript.