Using Top Level Await In Typescript: Simplifying Asynchronous Javascript Code

TypeScript code where you needed to use the “await” keyword outside of an asynchronous function? TypeScript 3.8 introduced a new feature called “Top Level Await” which allows you to use the “await” keyword at the top level of your code. Isn’t that exciting?

In this article, we will explore what exactly Top Level Await is, how it works, and the benefits and limitations of using it in TypeScript.

What is Top Level Await in TypeScript?

Have you ever encountered situations where you wanted to use the ‘await’ keyword at the top level of your TypeScript code? Well, you’re in luck because TypeScript now supports top level await!

In traditional JavaScript and TypeScript, the ‘await’ keyword can only be used within an ‘async’ function. This means that you need to wrap your code in an ‘async’ function and call it to use the ‘await’ keyword. However, this can sometimes lead to messy and unoptimized code. With the introduction of top level await TypeScript, you can now use the ‘await’ keyword directly at the top level of your code without the need for an ‘async’ function. This makes your code cleaner, more readable, and easier to maintain.

Top level await allows you to write asynchronous code in a synchronous manner. It provides a convenient way to handle Promises and async/await syntax without the need for additional boilerplate code. When using top level await, you can directly await a Promise or any other asynchronous operation in the global scope. This means that you don’t need to wrap your code in an ‘async’ function or use an immediately invoked async function expression.This feature is especially useful when you want to perform initialization tasks or fetch data from an API before your application starts running. Instead of chaining multiple ‘await’ calls or nesting them within ‘async’ functions, you can now simply use top level await to await the promises and continue with the execution of your code.

However, it’s important to note that top level await is currently an experimental feature in TypeScript and may not be supported by all JavaScript runtime environments or transpilers. It is recommended to check the compatibility of the runtime environment or transpiler you are using before using this feature in production code.

How Does Top Level Await Work?

Top level await changes this by allowing you to use await at the top level of your module, meaning you can use it outside of any function or block scope. This way, you can directly await a promise without having to wrap your code in an async function.

When you use top level await in TypeScript, the compiler automatically wraps your code in an immediately invoked async function. This function encapsulates your top level await expression and executes it synchronously, blocking the execution of any following statements until the promise is resolved or rejected.

For example, let’s say you have a module that needs to fetch some data from an API. With top level await, you can write code like this:

import { fetchData } from './api';

const data = await fetchData();

console.log(data);

In this example, the fetchData function returns a promise that resolves to some data. Instead of using a traditional then callback, we can directly await the promise at the top level of our module and assign the result to the data variable. The console log statement will only execute once the promise is successfully resolved.

Top level await also allows you to use try-catch blocks to handle promise rejections. If the awaited promise is rejected, the catch block will be executed just like in regular async/await code.

It’s important to note that top level await is only available in modules that are marked as ES modules (i.e., have the module flag set to “ESNext” or “ES2020”). If you’re using CommonJS modules, you won’t be able to directly use top level await.

Furthermore, you need to ensure that your runtime environment supports top level await.

Benefits of Using Top Level Await in TypeScript

Top level await offers several benefits for asynchronous programming in TypeScript:

Simplified Syntax:

With top level await, you can directly use “await” at the outermost level of your code, eliminating the need for async function wrappers. This results in cleaner and more readable code.

Improved Error Handling:

Using top level await allows you to handle errors in a centralized manner. Instead of scattering error handling throughout multiple async functions, you can use a try/catch block at the top level to catch and handle errors from the awaited operations.

Easier Debugging:

Top level await simplifies the debugging process by providing a straightforward way to pause the execution and inspect the state of the code. This can be especially useful when working with complex asynchronous operations.

Faster Development:

By eliminating the need for explicit async function wrappers, top level await reduces boilerplate code, resulting in faster development time. It allows you to directly use “await” wherever needed, reducing the cognitive load of managing async functions.

Limitations of Top Level Await in TypeScript

While using top level await in TypeScript can be beneficial, it is important to note that there are also some limitations to be aware of. These limitations may impact your decision to use top level await in certain scenarios.

1. Compatibility

One major limitation of top level await in TypeScript is that it is only supported in environments that have native support for ECMAScript modules and top level await. This means that if you are working with older versions of JavaScript or in environments that do not support these features, you will not be able to use top level await.

Additionally, not all browsers and JavaScript engines have implemented top level await yet, which can restrict where you can use this feature. It is important to check the compatibility of your target environments before deciding to use top level await in your TypeScript code.

2. Module Systems

Another limitation of top level await is its interaction with module systems. When using top level await, your code must be within an ECMAScript module, which may require additional configuration and setup, especially if you are working with existing codebases that use different module systems.

For example, if you are using CommonJS modules in Node.js, you would need to convert your code to ECMAScript modules in order to use top level await. This can be a time-consuming process, and may not be feasible in all projects.

3. Debugging and Tooling

Debugging and tooling support for top level await in TypeScript can also be limited. Some IDEs and development tools may not fully understand and handle top level await, which can lead to issues with code completion, type checking, and other features.

Similarly, when debugging code that uses top level await, the debugger may not be able to step through the asynchronous code in the expected manner. This can make debugging more challenging and time-consuming.

4. Code Complexity

Using top level await can also introduce additional complexity to your code. Especially if you are not familiar with asynchronous programming concepts. It may require a deeper understanding of promises, async/await syntax, and potential pitfalls such as handling errors and cancellations.

This added complexity can make your code harder to read, understand, and maintain, especially for developers. It is important to consider the skill level and experience of the developers who will be working with the codebase.

5. Performance Implications

Lastly, it is worth noting that using top level await can have performance implications, especially in certain scenarios. Top level await blocks the execution of the module until the awaited promise is resolved. It can potentially delay the loading and execution of other modules or scripts that depend on it.

This can lead to slower application startup times and decreased performance, especially in large codebases with complex dependencies. It is important to carefully consider the performance impact of using top level await in your specific use cases.

Conclusion

While top level await can be a powerful feature in TypeScript, it is important to be aware of its limitations. By understanding these limitations, you can make an informed decision and ensure that your codebase remains maintainable and performant.