Wait for Async Task to Finish
I'm Interacting with a Third-Party Javascript Library Where Some Function Calls Are Asynchronous. Instead of Working the Asynchronous Logic into My...
I'm interacting with a third-party JavaScript library where some function calls are asynchronous. Instead of working the asynchronous logic into my application, I preferred to write synchronous wrappers to those async calls. I know, I know, it's terrible design, but this is a demo project with very high chance of being rewritten entirely. I need something to show the team the concept, not really having to worry performance, yet.
Here's what I wanna do:
function sync_call(input) {
var value;
// Assume the async call always succeed
async_call(input, function(result) {value = result;} );
return value;
}
I tried the jQuery's deferred and promise but it seems to be aiming at the async design pattern. I want to use the synchronous pattern in my code.
3 Answers
This will never work, because the JS VM has moved on from that async_call and returned the value, which you haven't set yet.
Don't try to fight what is natural and built-in the language behaviour. You should use a callback technique or a promise.
function f(input, callback) {
var value;
// Assume the async call always succeed
async_call(input, function(result) { callback(result) };
}
The other option is to use a promise, have a look at Q. This way you return a promise, and then you attach a then listener to it, which is basically the same as a callback. When the promise resolves, the then will trigger.
How about calling a function from within your callback instead of returning a value in sync_call()?
function sync_call(input) {
var value;
// Assume the async call always succeed
async_call(input, function(result) {
value = result;
use_value(value);
} );
}
Here is a working example of how:-
function testAsync(){
return new Promise((resolve,reject)=>{
//here our function should be implemented
setTimeout(()=>{
console.log("Hello from inside the testAsync function");
resolve();
;} , 5000
);
});
}
async function callerFun(){
console.log("Caller");
await testAsync();
console.log("After waiting");
}
callerFun();
Outputs:
Caller
Hello from inside the testAsync function
After waiting
To make it more complete, error handling should be added (deal with the reject() case).
See here for other examples:
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