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4-expressjs.md

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Express JS

  • Backend web framework for Node.js;
  • Used to develop web applications and APIs;
  • Based on middleware;

Middleware

  • A middleware is a function that has access to the request and response objects, and the next middleware in the application's request-response cycle.
  • Performs the following tasks:
    • Execute any code;
    • Change the request and response objects;
    • End the request-response cycle;
    • Call the next middleware in the stack.

express()

To create an Express application, we use the express() function:

const express = require('express');
const app = express();

Methods

  • express.json([options]): parses incoming requests with JSON payloads;
  • express.Router([options]): creates a new router object;
  • express.static(root, [options]): serves static files from the specified root directory;
  • express.text([options]): parses incoming requests with text payloads;
  • express.urlencoded([options]): parses incoming requests with URL-encoded payloads;

Application

The app object conventionally denotes the Express application. Create it by calling the top-level express() function exported by the Express module.

A application object has methods for:

  • Routing HTTP requests;
  • Configuring middleware;
  • Rendering HTML views;
  • Registering a template engine;

Methods

  • app.all(path, callback [, callback ...]): matches all HTTP verbs;
  • app.get(name): returns the value of setting name;
  • app.listen([port[, host[, backlog]]][, callback]): binds and listens for connections on the specified host and port;
  • app.METHOD(path, callback [, callback ...]): matches METHOD HTTP verb;
  • app.render(view, [locals], callback): renders a view and writes the result to the response;
  • app.route(path): creates a new route with the specified path;
  • app.set(name, value): sets the value of setting name to value;
  • app.use([path,] callback [, callback ...]): mounts the specified middleware function or functions at the specified path.

Request

The req object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on.

Properties

  • req.app: the Express application;
  • req.body: parsed body;
  • req.cookies: cookie parser;
  • req.hostname: hostname derived from the Host HTTP header;
  • req.ip: remote address of the request;
  • req.method: request method;
  • req.params: route parameters;
  • req.path: path part of the request URL;
  • req.protocol: request protocol string;
  • req.query: query string parameters;
  • req.res: the response object;
  • req.route: the current route.

Response

The res object represents the HTTP response that an Express app sends when it gets an HTTP request.

Methods

  • res.cookie(name, value [, options]): sets cookie name to value;
  • res.clearCookie(name [, options]): clears the cookie name;
  • res.end([data][, encoding][, callback]): ends the response process;
  • res.get(field): returns the value of HTTP header field;
  • res.json([body]): sends a JSON response;
  • res.redirect([status,] path): redirects to the URL derived from the specified path, with specified status, a positive integer that corresponds to an HTTP status code;
  • res.render(view [, locals] [, callback]): renders a view and writes the result to the response;
  • res.req: the request object;
  • res.send([body]): sends the HTTP response;
  • res.sendStatus(statusCode): sets the response status code to statusCode and sends its string representation as the response body.
  • res.set(field [, value]): sets the HTTP header field to value, merging with the existing header field if it exists.

Router

A router object is an isolated instance of middleware and routes. You can think of it as a “mini-application,” capable only of performing middleware and routing functions. Every Express application has a built-in app router.

Methods

  • router.all(path, callback [, callback ...]): matches all HTTP verbs;
  • router.METHOD(path, callback [, callback ...]): matches METHOD HTTP verb;
  • router.use([path,] callback [, callback ...]): mounts the specified middleware function or functions at the specified path.