User-Defined Type Guards in TypeScript

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In TypeScript, one of the powerful features is the ability to define your own type guards. A type guard is a function that allows the compiler to infer more precise information about the type of a variable at a specific point in the code. Custom type guards are particularly useful when dealing with union types and specific checks are needed to determine which type is being used. In this text, we will explore in detail how user-defined type guards work and how they can be implemented in TypeScript.


What are User-Defined Type Guards?


User-defined type guards are functions created by the developer to assist the TypeScript compiler in inferring types based on specific checks. These custom checks are used to refine union types and make the TypeScript type system more accurate.


When to Use User-Defined Type Guards?


User-defined type guards are useful when working with union types and additional checks are needed to accurately determine which type is being used in a specific context. This is particularly helpful when dealing with code that handles heterogeneous data, and different behaviors are expected depending on the type.


Implementing User-Defined Type Guards:


To create a user-defined type guard, you need to define a function that returns a boolean value. This function should have the return type 'value is Type', where 'value' is the name of the variable being checked, and 'Type' is the specific type you want to assert.


Let's consider an example to illustrate the implementation of a user-defined type guard:


interface Circle {
      kind: 'circle';
      radius: number;
}
interface Square {
      kind: 'square';
      sideLength: number;
}
type Shape = Circle | Square;
function isCircle(shape: Shape): shape is Circle {
      return shape.kind === 'circle';
}


In the above example, we have two interfaces: 'Circle' and 'Square', representing different geometric shapes. The union of the 'Circle' and 'Square' types is defined as 'Shape'. The function 'isCircle' is a user-defined type guard that checks if a 'Shape' object is of type 'Circle' based on the 'kind' property. If the value of 'kind' is equal to 'circle', the type guard returns 'true', indicating that the object is of type 'Circle'.


Using User-Defined Type Guards:


After defining a user-defined type guard, you can use it to refine types in various situations. TypeScript will automatically recognize the type guard and infer more specific types based on the performed checks.


Consider the following example:


function calculateArea(shape: Shape) {
     if (isCircle(shape)) {
            // Here, TypeScript knows that 'shape' is of type 'Circle'.
            return Math.PI * shape.radius * shape.radius;
      } else {
            // Here, TypeScript knows that 'shape' is of type 'Square'.
            return shape.sideLength * shape.sideLength;
      }
}


In the above example, the calculateArea function takes a parameter 'shape' of type Shape. By using the user-defined type guard isCircle, we can perform a specific check to determine whether 'shape' is a circle or a square. Based on this check, TypeScript is able to correctly infer the types within each if and else block, allowing us to perform appropriate operations for each type.


Conclusion:


User-defined type guards are a powerful tool in TypeScript that allows you to refine union types based on custom checks. They help make the type system more expressive and secure, enabling more precise inferences in situations where heterogeneous types are used. By creating your own type guard functions, you can ensure greater accuracy in handling union types and enhance the robustness of your TypeScript code.

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