node express: Issue with res.render() after writing file to disk

I'm writing a method that uses async/await and promises to write some JSON to a file and then render a pug template. But for some reason the code that writes the JSON conflicts with the res.render() method resulting in the browser not being able to connect to the server.

The weird thing is that I don't get any errors in the console, and the JSON file is generated as expected — the page just won't render.

I'm using the fs-extra module to write to disk.

const fse = require('fs-extra');
exports.testJSON = async (req, res) => {

    await fse.writeJson('./data/foo.json', {Key: '123'})
        .then(function(){
            console.log('JSON updated.')
        })
        .catch(function(err){
            console.error(err);
        });

    res.render('frontpage', {
        title: 'JSON Updated...',
    });
}

I'm starting to think that there is something fundamental I'm not getting that conflicts with promises, writing to disk and/or express' res.render method. It's worth noting that res.send() works fine.

I've also tried a different NPM module to write the file (write-json-file). It gave me the exact same issue.

UPDATE: So I'm an idiot. The problem has nothing to do with Express og the JSON file. It has to do with the fact that I'm running nodemon to automatically restart the server when files are changed. So as soon as the JSON file was saved the server would restart, stopping the process of rendering the page. Apologies to the awesome people trying to help me anyway. You still helped me get to the problem, so I really appreciate it!

Here's the actual problem:

The OP is running nodemon to restart the server whenever it see filechanges, and this is what stops the code from running, because as soon as the json file is generated the server restarts.

Efforts to troubleshoot:

It's going to take some trouble shooting to figure this out and since I need to show you code, I will put it in an answer even though I don't yet know what is causing the problem. I'd suggest you fully instrument things with this code:

const fse = require('fs-extra');
exports.testJSON = async (req, res) => {

    try {    
        console.log(`1:cwd - ${process.cwd()}`);
        await fse.writeJson('./data/foo.json', {Key: '123'})
            .then(function(){
                console.log('JSON updated.')
            }).catch(function(err){
                console.error(err);
            });

        console.log(`2:cwd - ${process.cwd()}`);
        console.log("about to call res.render()");    
        res.render('frontpage', {title: 'JSON Updated...',}, (err, html) => {
            if (err) {
                console.log(`res.render() error: ${err}`);
                res.status(500).send("render error");
            } else {
                console.log("res.render() success 1");
                console.log(`render length: ${html.length}`);
                console.log(`render string (first part): ${html.slice(0, 20}`);
                res.send(html);
                console.log("res.render() success 2");
            }
        });
        console.log("after calling res.render()");
     } catch(e) {
         console.log(`exception caught: ${e}`);
         res.status(500).send("unknown exception");
     }
}