A tiny Express response interceptor
Express-interceptor allows you to define a previous step before sending a response. This allows you to do anything you want with the response, such as processing, transforming, replacing, or logging it. Express-interceptor allows you to avoid calling next() over and over. Further more, you can avoid managing nested scopes. Using a declarative API, it’s simple to use and maintain.
Some use cases include:
- Transpile custom elements into standard HTML elements.
- Transform JSX or compile less files on the fly.
- Store statistics about responses.
- Set response headers based on tags in the response body.
- Dynamically inject live-reload scripts to HTML.
Install the package
npm i --save express-interceptor
Define your interceptor
varexpress=require('express');varcheerio=require('cheerio');varinterceptor=require('express-interceptor');varapp=express();varfinalParagraphInterceptor=interceptor(function(req,res){return{// Only HTML responses will be interceptedisInterceptable: function(){return/text\/html/.test(res.get('Content-Type'));},// Appends a paragraph at the end of the response bodyintercept: function(body,send){var$document=cheerio.load(body);$document('body').append('<p>From interceptor!</p>');send($document.html());}};})// Add the interceptor middlewareapp.use(finalParagraphInterceptor);app.use(express.static(__dirname+'/public/'));app.listen(3000);You're defining an interceptor that will be executed whenever the response contains html as Content-Type. If this is true, it will append a child at the end of the body. In other words, it will transform the response:
<html><head></head><body><p>"Hello world"</p></body></html>Into:
<html><head></head><body><p>"Hello world"</p><p>From interceptor!</p></body></html>See more examples. Also, you can debug express-interceptor actions using debug env variable DEBUG=express-interceptor.
isInterceptable()(required): is a predicate function where you define a condition whether or not to intercept a response. Returningtruebuffers the request, and proceeds callingintercept()as well asafterSend(). Typically, you want to check for this condition in theresobject in the definition of the middleware.intercept(body, send)(required): Parse the body as an encoded string. After processing the body, callsend(newBody)with the content to be sent back to the client.afterSend(oldBody, newBody): This method will be called after sending the response to the client – after thedone()callback in thesend()method is executed. This method would typically be used to cache something, log stats, fire a job, among other things.
express-hijackresponse (deprecated) Different API, using callbacks with no top down structure, more obtrusive to HTTP internals.
hijackresponse Attempting to solve same problems than
express-hijackresponse. Different API using pipes, recommended if you have problems with streaming or if you need more control over responses.tamper Similar functionality but different internals and API.
If your intercept method make calls to a database, or needs to make expensive transformations to the original response, you should take in account the time that will add to the response. You should define your isInterceptable method carefully, checking the type of the response, or even the route that was fired by the request, also you may consider implementing a cache strategy to get faster responses.
If you face any issue, don't hesitate to submit it here.
Please check CONTRIBUTING.md.
This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.