Is the Promise Constructor’s Executor Function Synchronous or Asynchronous?

Asynchronous Execution of Promise Constructor Callback

The Promise constructor accepts an executor function that defines the promise's fulfillment logic. One key question arises: is this executor function executed asynchronously or synchronously?

Detailed Answer

The behavior depends on the implementation of the Promise itself. However, the ES6 standard for Promises explicitly states that the fulfillment of a promise is always asynchronous.

Referring to the specification, we find that the executor function (in our case, the y function) is indeed executed synchronously (see step 10 of the specification). However, subsequent calls to .then() on the promise, such as promise.then(...), are always executed asynchronously (see step 8 of the "PerformPromiseThen" algorithm).

Implications

This asynchronous nature of .then() calls has several implications:

• If the executor function modifies any variables or performs any side effects, those changes may not be immediately visible to the .then() handler.
• .then() handlers can be used to schedule tasks that should be executed after the current event loop finishes.
• This asynchronous behavior ensures that promises can be used to chain multiple asynchronous operations and handle their results sequentially.

Example Code

Consider the following code snippet:

<code class="javascript">function y(resolve, reject) {
  console.log("Result");
  resolve();
}

var promise = new Promise(y);
promise.then(() => {
  console.log("Then handler executed after the synchronous execution of y");
});</code>

In this example, the y function is executed synchronously. However, the .then() handler is executed asynchronously once the event loop finishes. The output of this code will be:

Result
Then handler executed after the synchronous execution of y

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