stamp-ots.js

function saveOts(otsFilename, buffer) {
    fs.exists(otsFilename, fileExist => {
        if (fileExist) {
            console.log('The timestamp proof \'' + otsFilename + '\' already exists');
        } else {
            fs.writeFile(otsFilename, buffer, 'binary', err => {
                if (err) {
                    return console.log(err);
                }
                console.log('The timestamp proof \'' + otsFilename + '\' has been created!');
            });
        }
    });
}

const OpenTimestamps = require('javascript-opentimestamps');
var infoResult;
const file = Buffer.from('5468652074696d657374616d70206f6e20746869732066696c6520697320696e636f6d706c6574652c20616e642063616e2062652075706772616465642e0a','hex');
const detached = OpenTimestamps.DetachedTimestampFile.fromBytes(new OpenTimestamps.Ops.OpSHA256(), file);

const Context = require('./node_modules/javascript-opentimestamps/src/context.js');   // loading directly from node_modules... ew
const ctx = new Context.StreamSerialization();
detached.serialize(ctx);
var buffer = new Buffer(detached.getOutput());
var otsFilename = 'fed.ots';
saveOts(otsFilename, buffer);

see https://gist.github.com/RCasatta/805091255747610a1d7d02d5c3bef1d2

The problems were:

• #L24 ctx.getOutput() instead of detached.getOutput()
• No need to require the Context from a require, it is accessible with OpenTimestams.Context
• You are not calling the stamp function (which I suppose is the reason of the script)