Getting original request object during multiple asynchronous calls in nodejs-request - node.js

Question

I have multiple HTTP requests in a nodejs app that each returns a word of a sentence. The replies will come at different times, so I'm saving them in a dictionary, with the key being the original sentence's word index. Problem is, when I access the request object, I only get the last one.
var completed_requests = 0;
sentence = req.query.sentence;
sentence = "sentence to be translated"
responses=[];
words = sentence.split(" ");
for(j=0;j<words.length;j++){
var word = words[j];
var data={
word:word
};
var options = {
url: 'example.com',
form:data,
index:j
};
request.post(options, function(err,httpResponse,body){
options = options;
if(!err){
responses.push({options.index: body});
completed_requests+=1;
if(completed_requests==words.length){
var a="";
for(var k=0;k<words.length;k++){
a+=responses[k]+" ";
}
res.render('pages/index', { something: a });
}
}
else{
//err
}
});
}
Basically, when I access the object.index object, the object returned isn't the one used for the original request, but the last one (for some reason). How should I resolve this?

Answer 1

When we take a look at how the code is evaluated by JavaScript due to it's async nature in node.js the problem becomes obvious:
For the first word the loop for(j=0;j<words.length;j++){ is executed.
The value of j is assigned to options.index. For the loop run this options.index has now the value 0.
request.post(options, function(err,httpResponse,body){ is executed but the callback handler will be invoked later.
For the first word the loop for(j=0;j<words.length;j++){ is executed.
The value of j is assigned to options.index. options.index has now the value 1.
request.post(options, function(err,httpResponse,body){ is executed but the callback handler will be invoked later.
The problem becomes obvious now since no new options objects are created but the value of j is assigned to options.index in every loop run. When the first callback handler is invoked options.index has the value words.length - 1.
To fix the problem we will wrap creating the options object in a function executeRequest
var completed_requests = 0;
sentence = req.query.sentence;
sentence = "sentence to be translated"
responses=[];
words = sentence.split(" ");
for(j=0;j<words.length;j++){
var word = words[j];
var data={
word:word
};
function executeRequest(url, form, index) {
var options = {
url: url,
form: form,
index: index
};
request.post(options, function(err,httpResponse,body){
// options = options; Superfluous
if(!err){
responses.push({ [index]: body});
completed_requests+=1;
if(completed_requests==words.length){
var a="";
for(var k=0;k<words.length;k++){
a+=responses[k]+" ";
}
res.render('pages/index', { something: a });
}
}
else{
//err
}
});
}
executeRequest('example.com', data, j);
}
A good read about scoping and hoisting in JavaScript can be found here http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html

Answer 2

You need to use an async routine such as forEach or map, also I suggest you read up on the async nature of node to help understand how to handle callbacks for io.