Solving the Route.get() requires a callback function but got a [object Object] Error in Express

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This error can be perplexing, especially for beginners. Let's break down what causes this error and how you can fix it.

Understanding the Error

In Express, .get() on a Route object requires a function as a callback parameter. Typically, this callback is responsible for handling the request and sending a response. However, if the provided parameter is not a function but an object, Express throws the mentioned error.

For example, the following code will cause the error:

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Here, { key: 'value' } is an object, not a function. Hence, Express cannot execute it as a callback.

Common Causes

Mixing up Middleware and Route Handlers: One common mistake is confusing middleware with route handlers. As middleware functions can also take three parameters (req, res, next), it’s easy to mistakenly pass an object when middleware return values are not functions.

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Here, middleware() immediately invokes the function and returns an undefined or a non-function value instead of passing the function itself.

Incorrect Import Structure: If you have an improperly structured import of your handler, it could be another source of this error.

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Ensure that exampleHandler is actually a function.

Solutions

Properly Passing Middleware: Make sure you are passing the middleware function itself without invoking it immediately.

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Ensuring Correct Exports: When structuring your handlers, export the function directly.

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Conclusion