Understanding How to Run async Functions Synchronously in JavaScript

Explore how to execute an `async` function synchronously within another function without using `await`, along with best practices to avoid common pitfalls.
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How to Run an async Function Synchronously in JavaScript

Asynchronous programming in JavaScript allows for more efficient code execution, but it can also introduce complexities that confuse even experienced developers. One common question that arises is whether you can run an async function synchronously, particularly when it's the last action in a function. In this post, we'll delve into this topic and explore how to effectively manage async functions without losing control over execution flow.

The Problem: Executing Asynchronous Functions

Consider the following example where you have two asynchronous functions:

[[See Video to Reveal this Text or Code Snippet]]

In the code above, the purpose of the function g is to ensure that flag = true in function f runs immediately after doPreparation(). However, calling f() without await means that g will not wait for f to complete before it is done executing, potentially leading to unexpected behavior in your application.

Solution: Using return vs. await

Immediate Execution of Functions

You might be wondering if you can get around this situation. The answer is yes! To ensure that flag = true runs immediately after doPreparation(), without yielding control to the event loop, you can simply return the f() function call. This allows g() to finish only once f() has started executing, maintaining the desired flow of execution. Here’s how that looks:

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In this case, even though f() is asynchronous, the return keyword ensures that g() will not resolve until after f() has been invoked.

Alternative Approaches

You might also think of using await in the last line of the g function, and it would look like this:

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This achieves the same result but introduces the possibility of managing errors with try/catch, which is a good practice for error handling in asynchronous functions.

Important Notes

If the promise that f() returns has an undefined resolved value, it can also be rewritten to:

[[See Video to Reveal this Text or Code Snippet]]

This again ensures proper control flow, and common practice would suggest using await if you need your asynchronous tasks to complete before proceeding.

Common Pitfalls to Avoid

One key distinction to remember is that calling f without await does not achieve the same control flow as the previous examples:

[[See Video to Reveal this Text or Code Snippet]]

In this wrong approach, the promise from g() resolves before the promise from f() does, disconnecting the two execution contexts entirely. This can lead to unhandled promise rejections and difficulties in debugging.

Conclusion

To run an async function synchronously in JavaScript, focus on using return or await appropriately, especially in the last lines of your asynchronous functions. This ensures that you maintain control of your code execution flow and handle errors effectively, providing a smoother development experience. With the right understanding of JavaScript's asynchronous nature, you can avoid pitfalls and write efficient, reliable code.