expressjs doesn't wait till query is done - express

Question

I am using expressjs to retrieve data from elasticsearch and send back to my angular app at the front end. Currently I am facing a problem since expressjs doesn't wait until the query execution is finished. I searched for a solution for that and the community says use "Promise or Sync". But I cant figure out where should I use it. I tried to use it but I am getting errors.
This is where I am receiving the request from the frontend and calling the elasticsearch query for send the response.
api.post('/clsDependencies', (req, res) => {
classDependencies(req.body.className);
res.json(messages);
});
This the function for querying the elasticsearch.
function classDependencies(csName) {
let body = {
size: 20,
from: 0,
query: {
match: {
ClassName: {
query: csName
}
}
}
};
search('testclass', body)
.then(results => {
results.hits.hits.forEach((hit, index) => hit._source.dependencies.forEach(
function(myClass){
messages.push({text: myClass.methodSignature , owner: `\t${++nmb} -
${myClass.dependedntClass}`});
}))})
.catch(console.error);
};
Expected data gets initialized to the variable(messages) which I am trying to send back to the front end. But the variable doesn't get initialized at the time when response is send back. What Should I do to wait till the query execution finish before send back the data to frontend.
EDIT
messages is defined outside of both functions.
function classDirectory(className) {
let body = {
size: 20,
from: 0,
query: {
match: {
ClassName: {
query: className
}
}
}
};
return search('testclass', body).then(results => {
results.hits.hits.forEach((hit, index) =>
getDirectories(hit._source.JarFileName));
return messages;
})
.catch(function(err) {
// log the error, but keep the promise rejected
console.error(err);
throw err;
});
};
function getDirectories(jarName) {
let body = {
size: 20,
from: 0,
query: {
match: {
jarFileName: {
query: jarName
}
}
}
};
return search('testjar', body).then(results => {
results.hits.hits.forEach((hit, index) =>
messages.push({text: hit._source.jarFileName , owner: `\t${++nmb} -
${hit._source.directory}`})
);
return messages;
})
.catch(function(err) {
// log the error, but keep the promise rejected
console.error(err);
throw err;
});
};

Answer 1

The Javascript interpreter does not "block" when you make asynchronous calls. This has absolutely nothing to do with Express.
Your call to search() is non-blocking so while it's in process, classDependencies() returns and the rest of your code continues to run. This is the way asynchronous calls in Javascript work.
If you want to call res.json() when classDependencies() is done, then return a promise from it and call res.json() when that promise resolves.
You could do something like this:
api.post('/clsDependencies', (req, res) => {
classDependencies(req.body.className).then(messages => {
res.json(messages);
}).catch(err => {
res.status(500).send(something here);
});
});
function classDependencies(csName) {
let body = {
size: 20,
from: 0,
query: {
match: {
ClassName: {
query: csName
}
}
}
};
return search('testclass', body).then(results => {
let messages = [];
results.hits.hits.forEach((hit, index) => hit._source.dependencies.forEach(function(myClass) {
messages.push({
text: myClass.methodSignature,
owner: `\t${++nmb} - ${myClass.dependedntClass}`
});
}));
// make messages be the resolved value of the returns promise
return messages;
}).catch(function(err) {
// log the error, but keep the promise rejected
console.error(err);
throw err;
});
};

Answer 2

api.post('/clsDirectory', (req, res) => {
classDependency(req.body.className, res);
});
function classDependency(csName, cb) {
let body = {
size: 20,
from: 0,
query: {
match: {
ClassName: {
query: csName
}
}
}
};
search('testclass', body)
.then(results => {
results.hits.hits.forEach((hit, index) =>
hit._source.dependencies.forEach(
function(myClass){
messages.push({text: myClass.methodSignature , owner: `\t${++nmb} -
${myClass.dependedntClass}`});
}));
cb.json(messages);
})
.catch(console.error);
};