Скачать или смотреть How to Handle JavaScript Variables with async/await Outside Functions

Discover how to manage variables using `async/await` in JavaScript effectively, including export techniques and understanding module behavior.
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Handling JavaScript Variables with async/await Outside Functions

In modern JavaScript, async and await have become essential tools for writing cleaner asynchronous code. However, many developers encounter problems when trying to use them outside of function contexts, especially during variable declarations. In this guide, we'll tackle a common scenario that involves exporting a variable initialized with await and discuss the implications of importing it from other files.

The Problem: Syntax Errors with await

When dealing with asynchronous operations in JavaScript, you might come across a situation where you want to define a variable outside of an async function using await. The JavaScript engine throws a syntax error in such cases, stating that await cannot be used outside of an async function. For instance, the following code generates an error:

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Here, we want to initialize firebaseConfig with a value returned by an asynchronous operation, but directly using await in this context is not allowed.

Questions to Address

How can you define a variable with await and export it?

When the variable is imported from other files, is it created only once, or every time it is imported?

The Solution: Using Functions to Handle Initialization

The good news is that we can achieve our goal by restructuring our code to use an asynchronous function to initialize the variable. Here's how to do it effectively:

Step 1: Define the Function for Initialization

Instead of directly declaring the variable with await, we can define a function that initializes the variable:

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Step 2: Call the Function at Application Start

You will now need to call setFirebaseConfig when your application starts. By doing this, you ensure that firebaseConfig gets assigned the value returned by RemoteConfig.build():

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

Benefits of This Approach

Avoid Syntax Errors: Using a function encapsulates the await call inside an async context.

Single Initialization: This approach ensures that RemoteConfig.build() is called only once and firebaseConfig is initialized at the app start.

Clean Export: You can export firebaseConfig without syntax issues.

Understanding Module Import Behavior

When you import variables from a module in JavaScript, the behavior depends on how the module is structured. If you were to write firebaseConfig = RemoteConfig.build();, the function would execute only once when the module is evaluated for the first time. If the module is imported multiple times, the already evaluated value of firebaseConfig will be returned without re-executing the function.

Conclusion

In summary, you cannot use await outside of an async function in JavaScript for variable declarations. However, by restructuring your code to utilize an asynchronous function to initialize variables, you can circumvent the syntax issues and manage your application state effectively. Remember, taking the time to properly structure your code ensures it remains clean, maintainable, and error-free.

By following these steps, you'll be able to handle asynchronous operations and exports in JavaScript smoothly, paving the way for a more efficient codebase.