Express Response Hooks

Extend Express.js with response hooks, primarily aimed to manipulate the response before being sent to the client.


npm install express-response-hooks


const responseHooks = require('express-response-hooks');

// response hooks initialization

// register a middleware that adds a "Cache-Control" header whenever the status code changes
app.use(function (req, res, next) {
    // hook on "statusCode" changes
    res.hooks.on('statusCode', ([ statusCode ]) => {
        if (req.method.toUpperCase() === 'GET') && (statusCode < 400)) {
            res.set('Cache-Control', 'public, max-age=5, s-maxage=31536000');



Creates a middleware that adds hooks to the response object

Options object

  • plugName

    • Type: string
    • Default: "hooks"

    Controls under which key the hooks event emitter will be available in the response object. Default to res.hooks


The hooks EventEmitter that enables registration to the following events:

Event Name Trigger Arguments Array Trigger
“statusCode” res.statusCode status code (int) statusCode property is changed internally
“set” res.set() header name (string), header value (string) set() is called internally or explicitly
“send” res.send() body (int / object / string) send() is called internally (e.g., res.json() calls it twice) or explicitly

For example

res.hooks.once('statusCode', ([ statusCode ]) => {
  // called once when the res.statusCode is changed for the first time
res.hooks.on('res', ([ name, value ]) => {
  // called whenever res.res() is called internally by express to set default headers or explicitly

Data manipulation

Changing the values in the arguments array will change the arguments passed to the original wrapped property/function

For example

res.hooks.on('res', (args) => {
  const [ name, value ] = args;
  if (name === 'cache-control') {
    // change the value of the header
    args[1] = args[1].replace('public', 'private');


An object with references to the wrapped properties and functions for using them without triggering an event.

For example

res.hooks.on('set', ([ name, value ]) => {
  if (name === 'cache-control') {
    // add additional header without triggering another 'set' event
    res.hooks.bypass.set('cdn-cache-control', value);


  • You should take into consideration that Express internally calls some of the response functions right before the response gets sent to the client (e.g., adding default headers)
  • statusCode manipulation will not work if the hook was triggered by send()


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