Running asynchronous functions once within express

Question

Is this the correct way to have asynchronous functions only run once within express? I don't want this function to have to run on every route. I'm looking for an express-friendly way to do this.

var packageJson = false
app.use(function(req, res, next){
  if(req.packageJson) return next()
  if(packageJson){
    req.packageJson = packageJson
    return next()
  }
  return fs.readFileAsync("./package.json", "utf8")
  .then(JSON.parse)
  .then(function(data){
    packageJson = data
    req.packageJson = data
    return next()
  })  
})

Bergi · Accepted Answer · 2015-07-07 16:34:39Z

2

I wouldn't even run the asynchronous function from within the first call to that route, but just do

var packageJson = fs.readFileAsync("./package.json", "utf8").then(JSON.parse);
app.use(function(req, res, next){
  if (req.packageJson)
    return next();
  else
    packageJson.then(function(data){
      req.packageJson = data;
      return next();
    });
});

But don't forget proper error handling, if that packageJSON promise is rejected your route might hang infinitely.

edited Jul 7, 2015 at 16:34

answered Jul 7, 2015 at 16:27

Bergi

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4 Comments

robertklep Over a year ago

Since it's a JSON file I would probably use require() :-)

Bergi Over a year ago

@robertklep: Yeah, maybe, but require is not asynchronous. Oh, and it would do the caching for us as well, so this feels like cheating :-)

robertklep Over a year ago

But it's read into memory just once, after that the data is being served from memory which is basically the same as "storing" the data in a promise value (I agree somewhat with it being cheating though, it's not a generic solution).

Bergi Over a year ago

Indeed. I wanted to show how to do it with any generic asynchronous function. require might be the better option for this particular use case.

robertklep · Accepted Answer · 2015-07-07 16:37:17Z

1

Just for reference (this is not a generic solution, @Bergi's is), in this particular case you can use require(), which has two useful features:

it can load JSON files directly
it will cache the file data in memory

The middleware would look like this:

app.use(function(req, res, next){
  req.packageJson = require('./package.json');
  return next();
});

The first request will take a small performance impact, as require() is synchronous, but for subsequent requests the data will be read from memory automatically.

answered Jul 7, 2015 at 16:37

robertklep

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