How is it that deferred callbacks in JavaScript do not lead to race conditions?

Question

How is it that deferred callbacks in JavaScript do not lead to race conditions?

I came across this snippet from the IndexedDB documentation and wanted to write something similar:

var req;
var store = getStore();
req = store.count();
req.onsuccess = function(evt) {
  console.log("success: " + evt.result);
};
req.onerror = function(evt) {
  console.error("add error", this.error);
};

https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API/Using_IndexedDB

This made me curious about why JavaScript allows for the definition of callbacks after a call and how it prevents race conditions. Can someone shed some light on this?

How does JavaScript ensure that an async call is not executed before the callback is assigned?

Thank you!

javascript promise race-condition deferred deferred-execution

Answer 1

Answer №1

JavaScript operates on a single thread, meaning the sequence of execution for your current code is as outlined below:

store.count();              // initiates some asynchronous operations
req.onsuccess; req.onerror;  // attaches the callback functions
                            // additional code within the same function/scope may be executed

                            // once your code finishes, the next item in the event queue is processed

store.count();              // executes the asynchronous part
req.onsuccess();            // triggers the callback function after the async operation completes

Hence, the timing of attaching callbacks isn't crucial as long as they are linked before the current function concludes and the JavaScript event queue moves on to the next task.

Regarding your statement: In modern JS engines, parsing and execution times differ. The engine compiles the code before running it (with rare instances where an old interpreter-style fallback is utilized, typically not applicable in most scenarios).

Answer 2

JavaScript operates on a single thread, meaning the sequence of execution for your current code is as outlined below:

store.count();              // initiates some asynchronous operations
req.onsuccess; req.onerror;  // attaches the callback functions
                            // additional code within the same function/scope may be executed

                            // once your code finishes, the next item in the event queue is processed

store.count();              // executes the asynchronous part
req.onsuccess();            // triggers the callback function after the async operation completes

Hence, the timing of attaching callbacks isn't crucial as long as they are linked before the current function concludes and the JavaScript event queue moves on to the next task.

Regarding your statement: In modern JS engines, parsing and execution times differ. The engine compiles the code before running it (with rare instances where an old interpreter-style fallback is utilized, typically not applicable in most scenarios).

How is it that deferred callbacks in JavaScript do not lead to race conditions?

Answer №1

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