Resolving the React Error: Expected an assignment or function call and instead saw an expression

Discover how to fix a common React error related to improper JavaScript expressions in JSX. Follow our step-by-step guide to prevent this issue in your projects!
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Resolving the React Error: Expected an assignment or function call and instead saw an expression

If you're developing with React and have encountered the error message "Expected an assignment or function call and instead saw an expression", you're not alone. This issue often arises when there's a misunderstanding of how to properly structure JavaScript code within JSX. In this guide, we will explore the root cause of this error and provide a clear, step-by-step guide to resolve it.

Understanding the Problem

Suppose you are trying to display information returned from an API and want to filter messages based on a condition (in this case, checking if the sentFromId has a length of 7). You may have structured your map iteration like this:

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

The error arises because you are attempting to use JSX inside an if statement without a proper return. The expression needs to result in a valid return value to be rendered by React.

Solution Strategy

Let's break down the solution into easy-to-understand steps:

Step 1: Understand JSX in Conditional Statements

In JavaScript, when using an arrow function, if you want to define a body with multiple lines, you must wrap it in curly braces {} and explicitly return the value you want. In your case, you were trying to return JSX from the if statement, but it was not properly set up.

Step 2: Correct the Code

Instead of using parentheses () for the JSX within your if statement, use a return within curly braces {}. This ensures it's treated as a proper instruction.

Here’s how you can revise the code:

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

Explanation of Changes

Use of return: By adding a return statement inside the if block, you are now correctly instructing React to render the JSX if the condition is true.

String Conversion: Since sentFromId may be a number, it's a good practice to convert it to a string using .toString(), ensuring that the length property works correctly.

Handling non-matching conditions: Adding return null; after the if statement ensures that when the condition is false, React still expects a return value.

Conclusion

The error "Expected an assignment or function call and instead saw an expression" can be easily resolved by understanding how to manage JavaScript expressions within JSX. By structuring your if statements correctly with a return, you'll keep your code functional and error-free.

For any React developer, mastering these nuances is vital for writing clean and effective code. Now you’re equipped with the knowledge to troubleshoot and refine your React applications!