I have asynchronous query in Node.js. Variable sq3 is a connection variable.
For example something like this:
for (var i in res) {
if (i == 1) {
sq3.query("SELECT * from students;",
function (err, res) {
if (err) {
throw err;
} else {
if (res.length == 1) {
//do something
} else {
//break for
}
}
});
sq3.end();
}
}
How can I break from callback function? Thanks
Just do it like this, using recursion instead of loops. Not only does this allow you to achieve the logic you want. It also doesn't spin up a bunch of async requests at once. They execute in turn, but asynchronously, so it's still performant.
function lookatEntireResponse(res) {
function lookAtItemInResponse(item) {
if(item == 1) {
sq3.query("SELECT * from students;",
function(err, res) {
if (err)
{
throw err;
}
else
{
if(res.length==1)
{
doSomething(item);
lookAtItemInResponse(res.shift());
}
else
{
//just don't call the next lookAtItemInResponse function, effectively same thing as "break;"
}
}
});
sq3.end();
} else {
lookAtItemInResponse(res.shift());
}
}
lookAtItemInResponse(res.shift());
}
You can consider throttling simultaneous requests with similar logic (say allowing 10 such requests per lookAtItem call. This way you can achieve a hybrid of the two methods, and then just optimize the number of simultaneous requests for performance. The Async library makes stuff like this easier.
In your code fragment, you can't break from the within the callback function. In node.js, callback functions run at an unspecified later time on the same thread. This means but the time you callback function executes, the for loop has long since finished.
To get the effect you want, you need to restructure you code quite significantly. Here's an example of how you could do it (untested!!). The idea is to keep calling doSomething() with the list of items, shrinking it by one element each time until the desired result is achieved (your break condition).
function doSomething(res)
{
while (res.length > 0)
{
i = res.shift(); // remove the first element from the array and return it
if (i == 1)
{
sq3.query("SELECT * from students;",
function(err, res) {
if (err)
{
throw err;
}
if (res.length==1)
{
//do something
// continue with the remaining elements of the list
// the list will shrink by one each time as we shift off the first element
doSomething(res);
}
else
{
// no need to break, just don't schedule any more queries and nothing else will be run
}
});
sq3.end();
break; // break from the loop BEFORE the query executes. We have scheduled a callback to run when the query completes.
}
}
}
res variable passed in to the original function is then reused as a parameter to the query callback function. I didn't notice the same variable name was being reused, so my code won't above work as it is. I'm not sure what the OP was trying to achieve, but my basic solution of recursively shifting through the list still holds!
for (var i = 0; i < res.length; i++) {
if (i == 1) {
sq3.query("SELECT * from students;",
function (err, res) {
if (err) {
throw err;
} else {
if (res.length == 1) {
//do something
} else {
i = res.length
}
}
});
sq3.end();
}
}
