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Express Response Hooks

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

Installation

npm install express-response-hooks

Usage

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

// response hooks initialization
app.use(responseHooks());

// 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');
        }
    });
);

API

require('express-response-hooks')([options])

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

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');
  }
});

res.hooks.bypass

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);
  }
});

Notice

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()