NodeJS callback - Access to 'res' - node.js

Question

I am using the Express framework and I have the following in one of my route files:
var allUsersFromDynamoDb = function (req, res) {
var dynamodbDoc = new AWS.DynamoDB.DocumentClient();
var params = {
TableName: "users",
ProjectionExpression: "username,loc,age"
};
dynamodbDoc.scan(params, function (err, data) {
if (err) {
console.error("Unable to query. Error:", JSON.stringify(err));
res.statusCode = 500;
res.send("Internal Server Error");
} else {
console.log("DynamoDB Query succeeded.");
res.end(JSON.stringify(data.Items));
}
});
}
I am using the above function in one of my routes:
router.get('/users', allUsersFromDynamoDb);
Now the callback that I am defining while making a call to the "scan" on dynamodbDoc can be pretty useful if defined as a separate function. I can re-use that for some of my other routes as well.
But how can I can still get access to the "res" inside this new function?
I think I should be using "closure" but I can't seem to get it exactly right. I think I would need to maintain the signature of the new callback function to expect 2 params, "err" and "data" as per the following page:
http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/DynamoDB/DocumentClient.html#scan-property
Any ideas on how this can be done?

Answer 1

You can use that function as middleware of every routes you want http://expressjs.com/en/guide/using-middleware.html
The new route with the middleware:
var middlewares = require('./middlewares'),
controllers = require('./controllers');
router.get('/users', middlewares.allUsersFromDynamoDb, controllers.theRouteController);
The middleware (middlewares.js) where you pass your data to req so you can use that data everywhere you have req:
exports.allUsersFromDynamoDb = function (req, res, next) {
var dynamodbDoc = new AWS.DynamoDB.DocumentClient();
var params = {
TableName: "users",
ProjectionExpression: "username,loc,age"
};
dynamodbDoc.scan(params, function (err, data) {
if (err) {
console.error("Unable to query. Error:", JSON.stringify(err));
next("Internal Server Error");
} else {
console.log("DynamoDB Query succeeded.");
req.dataScan = JSON.stringify(data.Items);
next();
}
});
};
And finally the controller (controllers.js):
exports.theRouteController = function (req, res) {
// Here is the dataScan you defined in the middleware
res.jsonp(req.dataScan);
};

Answer 2

Based on Michelem's answer here I tried something which makes things a bit cleaner and code more reusable:
var allUsersFromDynamoDb = function (req, res, next) {
var dynamodbDoc = new AWS.DynamoDB.DocumentClient();
var params = {
TableName: "users",
ProjectionExpression: "username,loc,age"
};
dynamodbDoc.scan(params, function (err, data) {
req.err = err;
req.data = data;
next();
});
}
Now I declare another function:
var processUserResults = function (req, res, next) {
if (req.err) {
console.error("Unable to query. Error:", JSON.stringify(req.err));
res.statusCode = 500;
res.send("Internal Server Error");
} else {
console.log("DynamoDB Query succeeded.");
res.end(JSON.stringify(req.data.Items));
}
};
And finally this:
router.get('/users', [allUsersFromDynamoDb, processUserResults]);
All I need to do in the original "function(err, data)" callback is always set 2 values:
req.err = err
req.data = data
And call next(). And processUserResults can similarly be used for other routes.
Still curious to find out if there are any other efficient solutions.