Passing Context objects through express does not work

[adamkdean]

I've been having an extended discussion on StackOverflow (link) with @sethkinast regarding passing Context objects through Express.

I have put together a very quick example of how to replicate this issue here: https://github.com/adamkdean/dust-test

But to save time, the script is this:

const path = require('path')
const hoffman = require('hoffman')
const express = require('express')
const request = require('request')
const duster = require('duster')
const dust = duster.dust

const app = express()
const base = dust.context({
  isBase: true,
  age: 26,
  people: [
    { name: 'exampleA' },
    { name: 'exampleB' }
  ]
})

app.set('views', path.join(__dirname, 'views'))
app.set('view engine', 'dust')
app.engine('dust', hoffman.__express())

app.get('/', function (req, res) {
  res.render('people', {})
})

app.get('/pojo', function (req, res) {
  res.render('people', {
    age: 26,
    people: [
      { name: 'exampleA' },
      { name: 'exampleB' }
    ]
  })
})

app.get('/context', function (req, res) {
  res.render('people', base.push({
    people: [
      { name: 'adam' }
    ]
  }))
})

app.listen(4000, function () {
  console.log('Visit http://localhost:4000 to see streaming!')
})

If you go to /pojo then the plain old JavaScript object renders fine (the template is {#people}<li>{name} aged {age}</li>{/people}), but if you go to /context then it doesn't render fine at all.

I think this would probably be a better place to discuss the issue.

[sethkinast]

Here's the issue: https://github.com/expressjs/express/blob/master/lib/application.js#L535

A new object, renderOptions, gets created. This object of course doesn't have Context as its prototype so Dust tries to shove it into a context.

It's very interesting that no one has ever run into this before. I would never have expected Express to mangle a context object passed to it. We can fix this by dropping the instanceof check on our end.

[sethkinast]

Hoffman even does the same (IMO incorrect) behavior in their stream method:

    res.stream = function(template, data, cb) {
      // flatten and add app and res locals data
      var options = {},
        stream;
      extend(options, res.app.locals);
      extend(options, res.locals);
      extend(options, data);
      ...

[sethkinast]

While we patch this you could manually stream your templates: https://github.com/linkedin/dustjs/blob/master/examples/streaming/app.js

You'll just have to define dust.onLoad yourself instead of your Express engine doing it for you.

[adamkdean]

Thanks Seth, appreciate the responses + effort involved. I thought I was doing something wrong, as like you say, very strange that this is the first time this has been run into. Then again, most examples exclusively use plain objects for their view models so perhaps that's why?

Anyway, thanks again. Will patch my version while I wait for your patch to hit!

[sethkinast]

Thanks for taking the time to report! I'm actually really fascinated that this has never been a problem before. I wonder if Express 4 changed the way that they merge options and locals.

[adamkdean]

Not a problem at all. Could not for the life of me understand what I was doing wrong. Thanks again, got everything working in my project now and no need for consolidate/hoffman now. 🎉

[sethkinast]

This is available as 2.7.4.