How to Fix an async function Executed Twice in React Components

Discover how to resolve the issue of an `async function` executing twice when called from a React component and understand the underlying behavior of async functions in React.
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Understanding the Problem: Async Functions in React

When working with React, developers sometimes encounter unexpected behavior that can be perplexing. A common issue arises when dealing with async functions. One such instance is when an async function seems to execute twice, causing confusion and frustration. This post will dissect this problem, using a specific example code snippet to illustrate how and why this occurs, and then provide a solution.

The Example Code

To illustrate the issue, consider the following code snippet:

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

In this code, TestAsyncComponent calls the asynchronous function doSomeThings, which is designed to log messages to the console. However, upon rendering this component within a React application, the console output reveals unexpected behavior:

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

Analyzing the Behavior

At first glance, it may seem that the doSomeThings function is not called twice, as the first log for React component appears only once. However, the logs indicate that the code following the await statement executes multiple times, which is quite puzzling.

To clarify the confusion, let's break down the output:

The initial calls from the module are straightforward, logging once for each step.

The critical misunderstanding arises from TestAsyncComponent, where the statements after await seem to execute twice.

Debugging the Issue

One approach to definitively determine where the issue lies is to utilize the debugger. Setting a breakpoint on the console log can help observe that it indeed fires twice during the React component's render cycle.

The Solution: Using useEffect

To prevent this unintended behavior, we can take advantage of the React hook useEffect. This hook allows us to run side effects, like calling an asynchronous function, once when the component mounts.

Implementation

Here’s how to modify the TestAsyncComponent:

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

Why This Works

With this modification:

The doSomeThings function is called only once when the component mounts, rather than every time it renders.

The second argument to useEffect ([]) signifies that this effect runs only once, simulating the behavior of componentDidMount.

Conclusion

In conclusion, React's behavior with async functions can sometimes lead to unexpected results, especially when dealing with renders and side effects. By understanding how and when to use hooks like useEffect, we can effectively manage these scenarios.

By following the guidance provided in this post, you can ensure your async functions execute as intended every time, without any surprises. Happy coding!