返回带有回调参数 function 的 function

Question

Consider this function:

export const catchAsync = (handler) => 
    (...args) => 
      ^^^^ why are these the parameters to handler, and not catchAsync?
        handler(...args).catch(args[2]);

In the first returned function, it says:

    (...args: [Request, Response, NextFunction]) => 

Now, to my knowledge, ...args would be exactly one thing: 'handler' It wouldn't be the parameters to the callback because those aren't the arguments to catchAsync, they are the arguments to the callback function 'handler'. Is there something I didn't learn about arguments of a callback being passed to the original function?

Thanks!

Answer 1

catchAsync is a function which returns another function. The outer function and the inner function each get passed different things. You would use this code something like the following:

const handlerWithCatch = catchAsync(someHandlerFn);
handlerWithCatch(someRequest, someResponse, someNextFn);

// Or on a single line:
catchAsync(someHandlerFn)(someRequest, someResponse, someNextFn)

On the insider handler will contain someHandlerFn , and args will be an array of [someRequest, someResponse, someNextFn]

Answer 2

Just re-read your question 1000 times.

As you mention ...args is not the argument of catchAsync. This function is how you implement the Strategy design pattern in Javascript.

catchAsync itself will return a function that you will call with req,res,next

catchAsync(handler)(req,res,next);