Allow async functions to return union type T | Promise
  • 11-Jun-2023
Lightrun Team
Author Lightrun Team
Allow async functions to return union type T | Promise

Allow async functions to return union type T | Promise

Lightrun Team
Lightrun Team

Explanation of the problem

When using async functions or methods in TypeScript, the return type must be the global Promise type. However, it is possible to explicitly declare a function’s return type as a union that includes Promise<T>. While this approach works when manually managing promises, it results in a compiler error when using the async/await syntax.

By defining a return type as T | Promise<T>, developers creating abstraction layers can have an abstract return type that can be handled in a type-safe manner without dictating implementation specifics to consumers. This improves developer ergonomics and allows for changes in implementation while still meeting the requirements of the abstraction layer.

Currently, this approach only works when developers explicitly manage promises. If a consumer of a function switches to a promise-based method, there is no need to change the explicit return type. However, if the consumer prefers using async/await syntax, a compiler error occurs due to the requirement of the global Promise type as the return type of an async function or method.

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Problem solution for Allow async functions to return union type T | Promise<T>

Two workarounds exist for this issue. One is to always return promises, even when dealing with non-async code. Another workaround is to use an implicit return type. While these workarounds resolve the issue, they introduce restrictions on the consumers of the abstraction layer and can leak implementation details.

This feature would be beneficial for developers building abstraction layers, such as middlewares, IoC containers, or ORMs, who want to provide an abstract return type that can include promises. An example of its use is in inversify-express-utils, where the invoked action can be either async or non-async, and the desired behavior remains consistent.

type ActionResponse<T> = T | Promise<T>;

function getCurrentUsername(): ActionResponse<string> {
  return 'Constant Username';

async function logResponse<T>(response: ActionResponse<T>): Promise<void> {
  const responseValue = await response;


The provided code demonstrates the usage of the suggested approach. Initially, the getCurrentUsername function has a return type of ActionResponse<string>, which can be a constant string or a promise. As the code evolves, the function can be changed to an async function that returns a Promise<string>, without modifying the return type. This behavior is handled transparently by functions like logResponse.

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) { = name; = email;

const user = new User("John Doe", "");
  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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