Скачать или смотреть How to Check if an Object Belongs to a Type in TypeScript

Learn how to determine if a string matches a specific TypeScript type using a custom type narrowing function.
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This video is based on the question https://stackoverflow.com/q/71649719/ asked by the user 'Y the tinker' ( https://stackoverflow.com/u/16741033/ ) and on the answer https://stackoverflow.com/a/71649850/ provided by the user 'David Deutsch' ( https://stackoverflow.com/u/724411/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Check if an Object Belongs to a Type in TypeScript

When working with TypeScript, you might encounter a situation where you need to verify if a particular value belongs to a defined type. Let's take a look at a straightforward example: you have a custom type called person, which can only be either 'man' or 'woman'. Now, suppose you have an input string x and want to check if it matches this type.

This can be a common requirement in programming, especially when working with dynamic data that may not always adhere to your defined types. Unfortunately, TypeScript's types are strictly for compile-time checks and don't exist during runtime. But, don’t worry – there’s a way around this with a custom function!

The Solution: Creating a Custom Type Narrowing Function

Since TypeScript does not provide built-in methods to check if a value matches a type at runtime, you'll need to construct your own type narrowing function. Let's go through how you can achieve this step by step.

Step 1: Define the Type

First, ensure you have your type defined. In this instance, we define a type named person:

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

Step 2: Implement the Type Narrowing Function

Next, you need a function that will accept input as a string and check if it matches the person type. Here's how you can do it:

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

The function isPerson takes a parameter x of type string.

It checks if x is equal to 'man' or 'woman' and returns true if either of those conditions hold.

Step 3: Utilizing the Type Narrowing Function

You can use this function within your code to ensure type safety when passing parameters. Here’s an example:

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

Explanation of the Example:

In funcThatTakesAString, trying to pass x directly to funcThatTakesAPerson results in a compile error since x is a string.

However, with the isPerson(x) check, if it returns true, TypeScript recognizes that x is indeed of type person, allowing you to call funcThatTakesAPerson(x) without any errors.

Conclusion

In conclusion, while TypeScript does not offer an out-of-the-box solution to check types at runtime, by implementing a custom type narrowing function as shown above, you can effectively ensure type safety in your applications. This practice not only enhances the reliability of your code but also makes it easier to maintain and scale.

By using this approach, you're leveraging TypeScript's strengths, ensuring that your code adheres to the types you’ve defined while avoiding potential runtime errors. Happy coding!