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()