When Does a Promise\'s Constructor Callback Execute?

The Timing of Promise Constructor Callback Execution

When creating a Promise object, a callback function (executor) is passed to the constructor. This callback function is responsible for initializing and fulfilling or rejecting the Promise. However, it may not be immediately clear when this callback is executed.

In the provided code snippet, the question arises: when is the doSomeWork() function called in relation to the creation of the Promise?

Answer:

According to the Promise specification and the MDN documentation, the executor callback is called synchronously immediately after the Promise is constructed. This means that doSomeWork() is executed as soon as the new Promise line is reached.

Additional Information:

This synchronous execution ensures that the executor callback is always called before other operations that may depend on it. For instance, if you were chaining multiple Promises or using all or race to combine Promises, the callbacks of those Promises would be guaranteed to be called in the correct order.

Furthermore, the synchronous call of the callback allows for immediate side effects from the Promise execution, if any.

Note:

It's important to understand that the asynchronous behavior of a Promise only takes effect after the executor callback has been called and the Promise has been created. Therefore, the code within the executor callback will execute synchronously, while any subsequent asynchronous operations will take place asynchronously.

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