Types for async functions do not handle `void` assignments like normal ones
  • 11-Jun-2023
Lightrun Team
Author Lightrun Team
Types for async functions do not handle `void` assignments like normal ones

Types for async functions do not handle `void` assignments like normal ones

Lightrun Team
Lightrun Team

Explanation of the problem

The code provided demonstrates different scenarios involving function return types and the use of void in TypeScript. The expected behavior is for all the scenarios to either result in an error or not. However, the actual behavior reveals a discrepancy in the handling of the “function returning void” rule when it comes to async scenarios.

In the code, we have several examples that illustrate this issue. First, there are assignments such as const w: void = ""; and const x: () => void = () => "";, where empty strings are assigned to variables with void type annotations. These assignments are currently allowed due to the “function returning void” rule.

However, when it comes to async scenarios, the behavior differs. For instance, the assignment const y: () => Promise<void> = async () => ""; results in an error. The async function returning an empty string is not considered assignable to the () => Promise<void> type, unlike the previous cases.

This inconsistency in handling the “function returning void” rule between regular functions and async functions leads to unexpected behavior. It is important to address this issue and ensure consistent behavior across all scenarios to maintain the integrity and predictability of TypeScript’s type system.


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Problem solution for Types for async functions do not handle `void` assignments like normal ones

The suggestion presented is to treat void as similar to unknown in terms of behavior, while still enforcing lint-level checks to prevent actual non-void returns in implementations where the expected return type is void. This proposal aims to maintain consistency and provide clearer guidelines for the usage of void in TypeScript. By aligning the behavior of void with that of unknown, it allows for better control over runtime errors and ensures that the type system accurately reflects the intentions of the programmer.

Another viewpoint suggests that the current duality between void and any in TypeScript is elegant and serves its purpose. The analogy presented likens any to a permissive type that allows any value, absolving the type system from responsibility for runtime errors. On the other hand, void is characterized as a type that allows arbitrary values to be assigned but prevents inspection of the actual value. This perspective appreciates the current design and the distinct roles played by void and any in TypeScript’s type system.

In summary, the responses reflect differing opinions on the behavior of void in TypeScript. One suggests aligning it with unknown while enforcing lint-level checks, while the other finds the existing duality between void and any elegant and meaningful. Ultimately, it is up to the TypeScript community and language designers to determine the best approach to handling void and ensure that it meets the overall goals and design principles of the language.


Other popular problems with Microsoft TypeScript

Problem: Incorrect Use of TypeScript Interfaces

TypeScript interfaces are a powerful tool for enforcing strict type checking in a codebase. However, incorrect use of interfaces can lead to problems with code accuracy and maintainability. For example, if an interface is defined with properties that are not used elsewhere in the code, it can be difficult to track down the source of an error later on.


To avoid this problem, it is recommended to make use of strict null checking and optional properties in interfaces. Additionally, be mindful of the properties and methods defined in an interface, and make sure that they are actually used elsewhere in the code. If an interface is no longer needed, it should be removed to prevent confusion and errors.

Problem: TypeScript Compilation Errors

TypeScript is a statically-typed language, which means that all type information is known at compile time. This can lead to compilation errors when code is written that violates TypeScript’s type system. For example, if a variable is declared with a type of string, and an attempt is made to assign a value of type number to it, a compile-time error will occur.


To resolve TypeScript compilation errors, it is important to carefully review the code and make sure that all variables are correctly declared with the correct type. In cases where a variable needs to be used with different types, a union type can be used to specify multiple types for the same variable. Additionally, the TypeScript documentation provides detailed information about the type system, and can be a valuable resource for resolving compilation errors.

Problem: Managing TypeScript Dependencies

Managing dependencies in a TypeScript project can be challenging, as different libraries and packages may have different versions and compatibility requirements. This can lead to conflicts and errors when attempting to use multiple libraries that have incompatible dependencies.


To resolve dependency management issues in a TypeScript project, it is recommended to make use of a package manager such as npm or yarn. These tools provide automated dependency management, and can help to prevent conflicts and errors when using multiple libraries and packages. Additionally, it is important to keep dependencies up-to-date, as newer versions may resolve compatibility issues and improve the overall stability of the project.

A brief introduction to Microsoft TypeScript

Microsoft TypeScript is a statically-typed, open-source programming language that builds on JavaScript. It is designed to provide optional type safety, improved tooling, and enhanced scalability to JavaScript code. TypeScript offers a language structure that is familiar to JavaScript developers, but with the added benefits of static type checking and enhanced tooling support.

TypeScript is designed to be compatible with existing JavaScript code and integrates seamlessly into many popular development environments and build tools. The language offers features such as class and interface definitions, type inference, and advanced type checking, making it easier for developers to write robust, maintainable code. TypeScript also includes a transpiler that can convert TypeScript code into equivalent JavaScript code, allowing developers to write TypeScript code that can run in any environment that supports JavaScript.

Most popular use cases for Microsoft TypeScript

  1. Large-scale web application development: TypeScript is well-suited for developing large-scale web applications, as it provides developers with the ability to write scalable, maintainable code. With its optional type checking, developers can catch type-related errors at compile time, making it easier to catch bugs and reduce the time spent debugging code. Additionally, TypeScript’s compatibility with existing JavaScript code allows developers to gradually adopt the language in their existing codebases, making it easier to transition to a statically-typed codebase.
class User {
    name: string;
    email: string;

    constructor(name: string, email: string) {
        this.name = name;
        this.email = email;

const user = new User("John Doe", "johndoe@example.com");
  1. Improved tooling support: TypeScript integrates well with modern development environments and build tools, making it easier for developers to write, manage, and maintain code. With TypeScript’s enhanced tooling support, developers can benefit from features such as code completion, refactoring, and debugging, which can help to increase developer productivity and reduce the time spent on manual code management tasks.
  2. Interoperability with JavaScript libraries: TypeScript is designed to be compatible with existing JavaScript code, making it easy for developers to integrate TypeScript with existing JavaScript libraries and codebases. Additionally, TypeScript provides a way to define type information for JavaScript libraries, making it easier to write TypeScript code that interacts with existing JavaScript libraries in a type-safe manner. This can help to reduce the time spent debugging and improve the overall stability of code.

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