Understanding the `never` Type in TypeScript

2.8 The never Type

In this section, we will delve into the never type in TypeScript, a unique type that represents values that never occur. Understanding never is crucial for mastering TypeScript’s type system, especially when dealing with functions that never return or throw errors. Let’s explore what the never type is, how it is used, and when you might encounter it in your TypeScript journey.

What is the never Type?

The never type in TypeScript is a special type that signifies the absence of a value. It is used to represent situations where a function never successfully completes. This can occur in two primary scenarios:

1. Functions that throw errors: These functions terminate execution by throwing an exception.
2. Functions with infinite loops: These functions run indefinitely without reaching an endpoint.

The never type is a subtype of every other type, but no type is a subtype of never (except never itself). This means that never can be assigned to any type, but no type can be assigned to never.

Functions that Throw Errors

One of the most common uses of the never type is in functions that throw errors. When a function throws an error, it does not return a value; instead, it halts execution. Let’s look at an example:

function throwError(message: string): never {
    throw new Error(message);
}

// Usage
throwError("This is an error message");

In this example, the throwError function is explicitly annotated with the never type because it always throws an error and never returns a value. This is a clear indication to anyone reading the code that this function will not complete normally.

Functions with Infinite Loops

Another scenario where the never type is applicable is in functions that contain infinite loops. These functions are designed to run indefinitely and never reach a return statement. Here’s an example:

function infiniteLoop(): never {
    while (true) {
        console.log("This will run forever");
    }
}

The infiniteLoop function is annotated with the never type because it contains an infinite loop that prevents it from returning a value.

TypeScript’s Use of never in Type Inference

TypeScript uses the never type in type inference to indicate unreachable code. When TypeScript determines that a particular code path will never be executed, it assigns the never type to that path. Consider the following example:

function example(value: string | number): string {
    if (typeof value === "string") {
        return "It's a string!";
    } else if (typeof value === "number") {
        return "It's a number!";
    } else {
        // TypeScript infers this block as `never`
        throw new Error("Unexpected value type");
    }
}

In this example, the else block is technically unreachable because the value parameter is constrained to be either a string or a number. TypeScript infers the type of this block as never, indicating that it should not be possible to reach this code.

When to Explicitly Annotate with never

While TypeScript can often infer the never type, there are situations where you might want to explicitly annotate a function with never for clarity and documentation purposes. Explicit annotations can help convey the intent of the code to other developers or future you. Here are some guidelines:

• Error-Throwing Functions: If a function is designed to always throw an error, annotate it with never to make its behavior clear.
• Infinite Loops: Use never for functions with infinite loops to indicate they do not return.
• Exhaustive Checks: In switch statements or conditional checks, use never to ensure all possible cases are handled. This is particularly useful when working with union types.

Cautionary Notes on Using never

While the never type is a powerful tool in TypeScript, it should be used with caution:

• Avoid Overuse: Do not overuse never in situations where it is not necessary. Reserve it for cases where a function genuinely does not return.
• Understand Type Inference: Rely on TypeScript’s type inference capabilities to determine when never is appropriate. Explicit annotations are not always needed.
• Code Clarity: Use never to improve code clarity, but ensure it does not make the code harder to read or understand.

Try It Yourself

Experiment with the never type by modifying the examples above. Try creating a function that uses a switch statement with exhaustive checks, and see how TypeScript infers the never type for unhandled cases.

Visualizing the never Type

To better understand how the never type fits into TypeScript’s type system, let’s visualize it using a type hierarchy diagram:

    graph TD;
	    A[any] --> B[string]
	    A --> C[number]
	    A --> D[boolean]
	    A --> E[object]
	    A --> F[undefined]
	    A --> G[null]
	    A --> H[never]

In this diagram, never is shown as a subtype of all other types, emphasizing that it can be assigned to any type, but no type can be assigned to never.

Summary

The never type is an essential part of TypeScript’s type system, representing functions that never return. It is used in error-throwing functions, infinite loops, and type inference for unreachable code. By understanding and using the never type correctly, you can write more robust and predictable TypeScript code.

Quiz Time!

By understanding the never type, you are better equipped to handle functions that do not return and ensure that your TypeScript code is both robust and predictable. Keep experimenting and exploring to deepen your understanding of TypeScript’s powerful type system!