const myLogger = function (req, res, next) )) What are Promises A promise is basically an advancement of callbacks in Node. Here we used Promise.all to easily wait for all promises in our array to be resolved or rejected (when a file is missing). The middleware function is assigned to a variable named myLogger. How do promises work JavaScript promises have three states: pending, resolved, and rejected. Promises, unlike typical callbacks, may be chained. If the promise is fulfilled, it is resolved otherwise, it is rejected. This function just prints “LOGGED” when a request to the app passes through it. A JavaScript promise represents an activity that will either be completed or declined. Here is a simple example of a middleware function called “myLogger”. One called myLogger that prints a simple log message, one called requestTime thatĭisplays the timestamp of the HTTP request, and one called validateCookies that validates incoming cookies. The remainder of this article will define and add three middleware functions to the application: Here is an example of a simple “Hello World” Express application. next will be called with either the rejected value or the thrown Error.
Starting with Express 5, middleware functions that return a Promise will call next(value) when they reject or throw an error. In case of completion, the promise is kept and otherwise, the promise is broken. A Promise in Node means an action which will either be completed or rejected. This is where Promises come into the picture. To resolve this issue we need to get rid of the callback functions whilst nesting. HTTP request argument to the middleware function, called "req" by convention. In Node.js world, this problem is called Callback Hell.
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