I was asked a question in an interview. below is the question.
const JsonFromHTTPCall = function(){
// make get request to some api url and return json object.
}
// code below is not editable
let result = JsonFromHTTPCall();
console.log("result ", result);
I am not finding a way to make console.log statement wait until I get the result from http call.
Please give me a way to solve it.
Thanks in advance.
Nodejs does not offer synchronous networking in any way. All built-in networking is asynchronous. Therefore, you cannot directly return a value from a function retrieved via networking. Instead, you need to communicate back the result either via a callback function, an event you trigger or a returned promise.
For a summary of this issue see this highly active question/answer:
How do I return the response from an asynchronous call?
There is a gross hack that involves using a synchronous child process and having it do the networking for you, but it's unlikely that is what they were asking for in your interview.
So, the main answer to the question is that "nodejs does not offer synchronous networking" and further "you cannot change an asynchronous result into a synchronous result". Therefore the proper way to code this is to use nodejs asynchronous coding techniques.
The cleanest way I know of to make http get calls is using a library such as request-promise() or my newer favorite got() and use the promise interface plus async/await to make a nice clean code path:
const got = require('got');
async function getSomeJSON(url) {
let data = await got(url).json();
console.log(data);
return data;
}
getSomeJSON(myURL).then(data => {
console.log("got my data");
}).catch(err => {
console.log(err);
});
Related
I am currently using the below server side rendering logic (using reactjs + nodejs +redux) to fetch the data synchronously the first time and set it as initial state in store.
fetchInitialData.js
export function fetchInitialData(q,callback){
const url='http://....'
axios.get(url)
.then((response)=>{
callback((response.data));
}).catch((error)=> {
console.log(error)
})
}
I fetch the data asynchronously and load the output in to store the first time the page loads using callback.
handleRender(req, res){
fetchInitialData(q,apiResult => {
const data=apiResult;
const results ={data,fetched:true,fetching:false,queryValue:q}
const store = configureStore(results, reduxRouterMiddleware);
....
const html = renderToString(component);
res.status(200);
res.send(html);
})
}
I have a requirement to make 4 to 5 API calls on initial page load hence thought of checking to see if there is an easy way to achieve making multiple calls on page load.
Do I need to chain the api calls and manually merge the response from different API calls and send it back to load the initial state?
Update 1:
I am thinking of using axios.all approach, can someone let me know if that is a ideal approach?
You want to make sure that requests happen in parallel, and not in sequence.
I have solved this previously by creating a Promise for each API call, and wait for all of them with axios.all(). The code below is
basically pseudo code, I could dig into a better implementation at a later time.
handleRender(req, res){
fetchInitialData()
.then(initialResponse => {
return axios.all(
buildFirstCallResponse(),
buildSecondCallResponse(),
buildThirdCallResponse()
)
})
.then(allResponses => res.send(renderToString(component)))
}
buildFirstCallResponse() {
return axios.get('https://jsonplaceholder.typicode.com/posts/1')
.catch(err => console.error('Something went wrong in the first call', err))
.then(response => response.json())
}
Notice how all responses are bundled up into an array.
The Redux documentation goes into server-side rendering a bit, but you might already have seen that. redux.js.org/docs/recipes/ServerRendering
Also checkout the Promise docs to see exactly what .all() does.developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
Let me know if anything is unclear.
You could try express-batch or with GraphQL is another option.
You also could use Redux-Sagas to use pure Redux actions to trigger multiple api calls and handle all of those calls using pure actions. Introduction to Sagas
I am new to nodejs so I have a basic question and this is my scanrio
I have a javascript client which is making a http request to a node server to read a value from the database.
Once the node server receives the request it makes a simple db call and returns the data to the client in the response, and this is where the problem is.
router.get('/state', function(req, res){
var result = dbServer.makeDBCall();//Before this line executes and returns the result the next line executes
res.send(result);
}
The database call from the node server is asynchronous, therefore before the result is returned the node server has already sent a blank response to the client. What is the standard/acceptable way of getting this achieved, I know I can block the node thread using async, but then the whole purpose of node is gone right?
It depends on what kind of database node module you are using.
Other than the standard callback approach, there are also the promise way. The pg-promise library is 1 of those kind.
See sample code:
this.databaseConnection.makeDBCall('your query...')
.then(function(dbResponse) {
// Parse the response to the format you want then...
res.send(result);
})
.catch(function(error) {
// Handle error
res.send(error.message);
});
#spdev : I saw 1 of your comments about you being worried about how Node actually knows who to reply the response to, especially when there are multiple requests.
This is a very good question, and to be honest with you - I don't know much about it as well.
In short the answer is yes, Node somehow handles this by creating a corresponding ServerResponse object when a HTTP request comes through. This object seems to have some smartness to tell the Nodejs network stack how to route itself back to the caller when it gets parsed as data packets.
I tried Googling a bit for an answer but didn't got too far. I hope the ServerResponseObject documentation can provide more insight for you. Share with me if you got an answer thanks!
https://nodejs.org/api/all.html#http_class_http_serverresponse
Try below code.
router.get('/state', function(req, res){
var result = dbServer.makeDBCall(function(err,result){
if(!err) {
res.send(result);
}
});
}
Hope this Help.
The dbServer.makeDBCall(); must have a callback that runs when the statement completes executing.
Something like -
dbServer.makeDBCall({query: 'args'}, function(err, result){
if (err) // handle error
res.send(result);
})
You return the response from db from that callback function.
Learn more about callback from here-
nodeJs callbacks simple example
https://docs.nodejitsu.com/articles/getting-started/control-flow/what-are-callbacks/
I have just started my journey with nodejs and would like to create a simple nodejs app that needs to:
- first request/get some initial data from via http,
- use received json to do another set of requests (some can be done in parallel, some needs to be executed first and data received will be used to create valid url).
Taking into account that nodejs is asynchronous and based on callbacks, I am wondering what is the best way to achieve this in order to have 'clean code' and not mess up with the code too much.
Thanks for any hints / guidelines, Mark
Maybe check out the Async library. Has a lot of built in functionality that seems to accomplish what you're looking for. Couple of useful ones right off the bat might be "async.waterfall" and "async.map".
async.waterfall
async.map
Agreed that this is subjective, in general the way to go is promises, there are native promises:
Native Promise Docs - MDN
For your particular question, imo, the npm module request-promise offers some great solutions. It is essentially a 'Promisified" version of the request module:
It will allow you to GET/POST/PUT/DELETE and follow up each request with a .then() where you can continue to do more calls like so:
-this code first GETS something from a server, then POSTS something else to that server.
function addUserToAccountName(url, accountName, username, password){
var options = assignUrl(url); // assignUrl is not in this code
request
.get(options) //first get
.auth(username, password)
.then(function(res) {
var id = parseId(res.data, accountName); //parse response
return id;
})
.then(function(id) {
var postOptions = Object.assign(defaultSettings, {url: url + id + '/users'})
request.post(postOptions) // then make a post
.auth(username, password)
.then(function(response) {
//console.log(response);
})
.catch(function(err) {
console.log((err.response.body.message));
})
})
}
You can just keep going with the .then() whatever you return from the previous .then() will be passed in to the function.
Request-Promise
I'm still learning the node.js ropes and am just trying to get my head around what I should be deferring, and what I should just be executing.
I know there are other questions relating to this subject generally, but I'm afraid without a more relatable example I'm struggling to 'get it'.
My general understanding is that if the code being executed is non-trivial, then it's probably a good idea to async it, as to avoid it holding up someone else's session. There's clearly more to it than that, and callbacks get mentioned a lot, and I'm not 100% on why you wouldn't just synch everything. I've got some ways to go.
So here's some basic code I've put together in an express.js app:
app.get('/directory', function(req, res) {
process.nextTick(function() {
Item.
find().
sort( 'date-modified' ).
exec( function ( err, items ){
if ( err ) {
return next( err );
}
res.render('directory.ejs', {
items : items
});
});
});
});
Am I right to be using process.nextTick() here? My reasoning is that as it's a database call then some actual work is having to be done, and it's the kind of thing that could slow down active sessions. Or is that wrong?
Secondly, I have a feeling that if I'm deferring the database query then it should be in a callback, and I should have the actual page rendering happening synchronously, on condition of receiving the callback response. I'm only assuming this because it seems like a more common format from some of the examples I've seen - if it's a correct assumption can anyone explain why that's the case?
Thanks!
You are using it wrong in this case, because .exec() is already asynchronous (You can tell by the fact that is accepts a callback as a parameter).
To be fair, most of what needs to be asynchronous in nodejs already is.
As for page rendering, if you require the results from the database to render the page, and those arrive asynchronously, you can't really render the page synchronously.
Generally speaking it's best practice to make everything you can asynchronous rather than relying on synchronous functions ... in most cases that would be something like readFile vs. readFileSync. In your example, you're not doing anything synchronously with i/o. The only synchronous code you have is the logic of your program (which requires CPU and thus has to be synchronous in node) but these are tiny little things by comparison.
I'm not sure what Item is, but if I had to guess what .find().sort() does is build a query string internally to the system. It does not actually run the query (talk to the DB) until .exec is called. .exec takes a callback, so it will communicate with the DB asynchronously. When that communication is done, the callback is called.
Using process.nextTick does nothing in this case. That would just delay the calling of its code until the next event loop which there is no need to do. It has no effect on synchronicity or not.
I don't really understand your second question, but if the rendering of the page depends on the result of the query, you have to defer rendering of the page until the query completes -- you are doing this by rendering in the callback. The rendering itself res.render may not be entirely synchronous either. It depends on the internal mechanism of the library that defines the render function.
In your example, next is not defined. Instead your code should probably look like:
app.get('/directory', function(req, res) {
Item.
find().
sort( 'date-modified' ).
exec(function (err, items) {
if (err) {
console.error(err);
res.status(500).end("Database error");
}
else {
res.render('directory.ejs', {
items : items
});
}
});
});
});
Using node.js, what is the best way to process a million items in an HTTP post request without blocking the server? My only guess is some sort of message queue, but I really have no idea.
You would want to use a lib like async.js to create non-blocking loops.
https://github.com/caolan/async
var async = require("async");
async.each(yourArrayOfThings, function(oneItem, callback) {
// do something
// ...
return callback(null);
}, function(err) {
// if any of the callbacks returned an error, err would equal that error
});
Give some more information on what your processing needs are, if this is not an applicable solution for you.