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To gauge the performance of our apps, we need to measure the response time of our API.

To do this with our Express app, we can use the response-time package.

Installation

response-time is available as a Node package. We can install it by running:

npm install response-time

Then we can import it by writing:

const responseTime = require('response-time');

responseTime([options])

We can set various options by passing an object into the optional options parameter.

It’ll create a middleware that adds a X-Response-Time header to responses.

The options object takes the following properties:

digits

The digits property is a fixed number of digits to include in the output, which is always in milliseconds. The default is 3.

header

The name of the header to be set. It defaults to X-Response-Time.

suffix

The suffix property is a boolean property to indicate if units of measure should be added to the output. The default value is true.

responseTime(fn)

The function creates a middleware that records the response time of a request and makes it available to our own function fn. fn has the signature (req, res, time) where time is a number in milliseconds.

Examples

Simple Example

We can use the responseTime package without any options or function passed in:

const express = require('express');  
const bodyParser = require('body-parser');  
const responseTime = require('response-time')  
const app = express();  
app.use(bodyParser.json());  
app.use(bodyParser.urlencoded({ extended: true }));

app.use(responseTime());

app.get('/', (req, res) => {  
  res.send('foo');  
});

app.listen(3000);

Then we get a X-Response-Time response header with a value like 0.587ms.

We can check the response header on an HTTP client like Postman.

Passing in Options

We can change the options for the header returned with the response. For example, we can write the following to change the number of digits sent:

const express = require('express');  
const bodyParser = require('body-parser');  
const responseTime = require('response-time')  
const app = express();  
app.use(bodyParser.json());  
app.use(bodyParser.urlencoded({ extended: true }));

app.use(responseTime({  
  digits: 5  
}))

app.get('/', (req, res) => {  
  req.id = 1;  
  res.send('foo');  
});

app.listen(3000);

Then we get a X-Response-Time response header with a value like 0.71987ms.

Passing in Our Own Function

We can pass a function into the responseTime function as follows:

const express = require('express');  
const bodyParser = require('body-parser');  
const responseTime = require('response-time')  
const app = express();  
app.use(bodyParser.json());  
app.use(bodyParser.urlencoded({ extended: true }));

app.use(responseTime((req, res, time) => {  
  console.log(`${req.method} ${req.url} ${time}`);  
}))

app.get('/', (req, res) => {  
  req.id = 1;  
  res.send('foo');  
});

app.listen(3000);

Then we get something like:

GET / 2.9935419999999997

from the console.log.

It’s useful if we want to manipulate the response time data or log it ourselves.

Conclusion

We can get the response time in the response header of a request with the response-time package.

It has a responseTime function which returns a middleware that we can use with the use method of express or express.Router() .

The function can either take an options object and or a function with the req, res, and time parameters to get the request, response, and response time respectively.