res.sendfile in Node Express with passing data along

Is there any way to redirect to an HTML file from a Node.JS application with something like: res.sendFile of express and pass a JSON data along to the html file?



You get one response from a given request. You can either combine multiple things into one response or require the client to make separate requests to get separate things.

If what you're trying to do is to take an HTML file and modify it by inserting some JSON into it, then you can't use just res.sendFile() because that just reads a file from disk or cache and directly streams it as the response, offering no opportunity to modify it.

The more common way of doing this is to use a template system that lets you insert things into an HTML file (usually replacing special tags with your own data). There are literally hundreds of template systems and many that support node.js. Common choices for node.js are Jade (Pug), Handlebars, Ember, Dust, EJS, Mustache.

Or, if you really wanted to do so, you could read the HTML file into memory, use some sort of .replace() operation on it to insert your own data and then res.send() the resulting changed file.


I know this is late but I wanted to offer a solution which no one else has provided. This solution allows a file to be streamed to the response while still allowing you to modify the contents without needing a templating engine or buffering the entire file into memory.

Skip to the bottom if you don't care about "why"

Let me first describe why res.sendFile is so desirable for those who don't know. Since Node is single threaded, it works by performing lots and lots of very small tasks in succession - this includes reading from the file system and replying to an http request. At no point in time does Node just stop what it's doing and read an entire from the file system. It will read a little, do something else, read a little more, do something else. The same goes for replying to an http request and most other operations in Node (unless you explicitly use the sync version of an operation - such as readFileSync - don't do that if you can help it, seriously, don't - it's selfish).

Consider a scenario where 10 users make a request for for the same file. The inefficient thing to do would be to load the entire file into memory and then send the file using res.send(). Even though it's the same file, the file would be loaded into memory 10 separate times before being sent to the browser. The garbage collector would then need to clean up this mess after each request. The code would be innocently written like this:

app.use('/index.html', (req, res) => {
   fs.readFile('../public/index.html', (err, data) => {

That seems right, and it works, but it's terribly inefficient. Since we know that Node does things in small chunks, the best thing to do would be to send the small chunks of data to the browser as they are being read from the file system. The chunks are never stored in memory and your server can now handle orders of magnitude more traffic. This concept is called streaming, and it's what res.sendFile does - it streams the file directly to the user from the file system and keeps the memory free for more important things. Here's how it looks if you were to do it manually:

app.use('/index.html', (req, res) => {


If you would like to continue streaming a file to the user while making slight modifications to it, then this solution is for you. Please note, this is not a replacement for a templating engine but should rather be used to make small changes to a file as it is being streamed. The code below will append a small script tag with data to the body of an HTML page. It also shows how to prepend or append content to an http response stream:

NOTE: as mentioned in the comments, the original solution could have an edge case where this would fail. For fix this, I have added the new-line package to ensure data chunks are emitted at new lines.

const Transform = require('stream').Transform;
const parser = new Transform();
const newLineStream = require('new-line');

parser._transform = function(data, encoding, done) {
  const str = data.toString().replace('</body>', '<script>var data = {"foo": "bar"};</script></body>');

// app creation code removed for brevity

app.use('/index.html', (req, res) => {
    res.write('<!-- Begin stream -->\n');
    .on('end', () => {
        res.write('\n<!-- End stream -->')

Well, it's kinda old, but I didn't see any sufficient answer, except for "why not". You DO have way to pass parameters IN static file. And that's quite easy. Consider following code on your origin (using express):

    let data = fs.readFileSync('yourPage.html');
    //else - 404

Now for example, just set a cookie, in yourPage.html, something like:

    var date = new Date();
    document.cookie = "yourCookieName='param1Place';" + 
    date.setTime(date.getTime() + 3600) + ";path=/";

And you can plainly pull content of uniqueData from yourCookieName wherever you want in your js


You only have one response you can return from the server. The most common thing to do would be to template your file on the server with nunjucks or jade. Another choice is to render the file on the client and then to use javascript to make an ajax call to the server to get additional data. I suppose you could also set some data in a cookie and then read that on the client side via javascript as well.


Why not just read the file, apply transformations and then set up the route in the callback?

fs.readFile(appPath, (err, html) => {
   let htmlPlusData = html.toString().replace("DATA", JSON.stringify(data));

   app.get('/', (req, res) => {

Note that you can't dynamically change data, you'd have to restart the node instance.


(Unless you want to template the html file to insert the json data into a script tag). You'll need to expose an api endpoint in express the send along the data to the page, and have a function on the page to access it. for example,

// send the html
app.get('/', (req, res) => res.sendFile('index'));

// send json data
app.get('/data', (req, res) => res.json(data));

Now on the client side you can create a request to access this endpoint

function get() {
  return new Promise((resolve, reject) => {
    var req = new XMLHttpRequest();'GET', '/data');
    req.onload = () => resolve(req.response);
 // then to get the data, call the function

get().then((data) => {
  var parsed = JSON.parse(data);
  // do something with the data


So arrow functions probably don't work client side yet. make sure to replace them with function(){} in your real code


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