How to use express-async-handler

Question

I am going through some code on github: https://github.com/linnovate/mean/blob/master/server/routes/user.route.js

But there is a portion of it I don't understand, that is:

router.route('/')
    .post(asyncHandler(insert));

On npm express-async-handler is described as:

Simple middleware for handling exceptions inside of async express routes and passing them to your express error handlers.

They go give an example of how to use the module, but it doesn't explain much.

So my questions are:

1. How is the insert function on line 12 called without parentheses?

2. What is the function of asyncHandler(), what would the code look like if you decide on not using it?

3. Normally when using router.route('/').post there are curly braceswhich follow. In this code I cant see any. So my question are: Is the async function insert part of the function body of router.route('/').post? and if not then why are there no curly braces?

4. What exactly is being exported here user.controller.js on line 14 (is it an object, a var...)? What is the advantage of exporting it this way? Why not just export the function insert()?

Thank you in advance.

Answer 1

How is the insert function on line 12 called without parentheses?

The insert function is not called here. It is passed to asyncHandler() as a function reference so it can be called later. It is asyncHandler() which is called immediately and that returns a new function that is passed to .post() as the request handler.

What is the function of asyncHandler(), what would the code look like if you decide on not using it?

This is a wrapper around insert that looks for a rejected promise returned from the function and, if found, calls next(err) automatically.

Normally when using router.route('/').post there are curly braceswhich follow. In this code I cant see any. So my question are: Is the async function insert part of the function body of router.route('/').post? and if not then why are there no curly braces?

I'm not sure what you mean by curly braces. .post() expects a function reference to be passed to it that will be called with a certain set of parameters when the defined route matches an incoming request. It can use as either of these:

// externally defined request handler function
router.route('/').post(someRequestHandlerFunc);

// inline defined request handler function
router.route('/').post(function(req, res, next) {
    // code here for the request handler
});

What exactly is being exported here user.controller.js on line 14 (is it an object, a var...)? What is the advantage of exporting it this way? Why not just export the function insert()?

I'm assuming the line 14, you're asking about is here. That's just exporting the insert function on the insert property of this modules exports. When you export a function, you don't use insert(). That calls the function immediately. You just refer to the function's name as insert to export a reference to the function that can be called later.

The reason to export is as a property of an object rather than just export the single function is to make the module extensible so it can export other things as different named properties.