I would like to expose a node.js server variable to the client. I am trying to get express-expose to work.
I am not sure how to initialise and use that library.

In the express-expose guide the usage doesn't make sense

var express = require('express');
var expose = require('express-expose');
app = expose(app);
app.expose(...);

how can I use app in 'expose(app') before it was initialised ?

I used

const app = expose(express());
app.expose('var some = "variable";','head');

but that doesn't seem to work either.

Can anyone supply me please with an example that initialises the express-expose library and exports var 'some' to the client ?

(By exposing to the client I mean it will be available to all my javascript files as a global variable so that I could do 'console.log(some);' and it would print its value)

Using Pug

Since you're using Express with Pug, you can just call the res.render function and give it the variable you want. Here's how to do that.

Let's say you have a template called index.pug, it may look like this:

html
 head
 title= title
 body
 h1= message

On your server, the code responsible for the rendering should look like this. In this case we're passing someVariable to the view that needs to be rendered.

const someVariable = 'hello world!';

app.get('/', function (req, res) 
 res.render('index', title: 'Hey', message: someVariable )
)

A note on express-expose

The express-expose library is not maintained and hasn't been updated in 5 years. My advice would be to just render your variable as described previously or just use AJAX requests.

A note on using a global variable

I am using the 'pug' view engine. It is passed to the view but I want
it as a global javascript variable.

A better idea is to use Template Inheritence to create "generic layouts" and extend them as needed. Sharing a global variable with file you're rendering will not update it magically on the front-end if you're changing it in the back-end. Even if the variable is global.

If you want to reflect "real-time changes" you should look into Socket.io.

This seems like a code smell... But regardless, there are cleaner ways to do it.

If you want to expose a data object, that is, an object just containing keys and values, you could embed it in the rendered webpage, make it available via some API endpoint that the client can acquire via an AJAX request, or expose a raw Javascript file that can be included via <script> (or I guess script(...) in pug).

If you want to expose a more complex Javascript object, such as a class or an object with function definitions, you could expose the Javascript file and include it via script(...).

However, you should be very careful exposing a file used by the server. If there are any vulnerabilities, they are now public. If there are any hard-coded passwords (which shouldn't be in code anyway), they are now public. Anything in this file is now public, so if you do indeed want to do this, be very careful about what code gets into this now client-side file.

If you need more details on how this could work, please comment :)

Edit: Also be aware that using the embedded and include methods would not allow the variable to be updated on-the-fly. If you need the client to track any changes to the data as it changes on the server, you will want to use AJAX. This is the best method, in my opinion. It offers you the most flexibility.

Another edit: Judging by the issues on the express-expose project, it seems not well supported and maybe has security issues. For now, I would avoid it. Maybe at a later time, it will be helpful and secure.

If you want to expose a variable value to the client from server , that never changes or updates, you can simply add a hidden input field in your template and add the value of it

In your EJS template file this would look like

<% if (data) %>
 <input type="hidden" value="<%= data %>" id="myGlobal"/>
<% %>

you must pass the data value in your res.render('template',data:"hello") function to the template.

You can access this variable value from client using a simple document.getElementById("myGlobal").Again don't pass any sensitive information in hidden fields.

If you want to pass sensitive information , implement an endpoint in your server with authentication and call the endpoint from client via an ajax call .This is also a best approach to update the value of the variable from client side.