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Discover how to create a strongly-typed function in TypeScript that accurately maps over an array of functions, returning the correct variadic tuple types.
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How to Assign Array# map Results to a Variadic Tuple Using TypeScript

Are you trying to create a strongly-typed function in TypeScript that maps over a homogeneous array of functions? If so, you might encounter some challenges when trying to ensure the return types match up correctly. In this guide, we'll explore a common problem and its solution, enabling you to work with variadic tuple types effectively.

The Problem

When using the Array# map method on a tuple of functions, TypeScript can throw errors related to the return types. For example, if you've defined your function like this:

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You might see an error that states:

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The Solution

To resolve this issue, we'll need to explicitly cast the returned value from Array# map to match our expected complex return type. Let’s break this down into clear steps.

Step 1: Define the Result Type

First, we need to create a type that represents the results of each function in the array. Here's how you can define it:

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This type checks each function in the tuple and maps it to its corresponding return type.

Step 2: Create the runAll Function

Now we can rewrite the runAll function to utilize the AllResults type we just defined. Here’s the updated function:

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Step 3: Using the Function

You can now use the runAll function with a tuple of functions. Here's an example:

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The inferred type for results is now [string, number, boolean], which accurately reflects the return types of the functions you passed in.

Conclusion

By explicitly casting the result from the Array# map method, we can ensure that our strongly-typed function yields the correct variadic tuple types as expected. While it might feel cumbersome to perform a cast, it ensures type safety within your TypeScript code.

Now that you know how to effectively assign Array# map results to a variadic tuple, you can harness the power of TypeScript’s type system to create safer and more robust applications. Happy coding!