The method by which JavaScript identifies when a Promise has resolved or rejected

Question

The method by which JavaScript identifies when a Promise has resolved or rejected

How does JavaScript determine when the state of myPromise has transitioned to "fulfilled" in the code provided below? In other words, what is the process that determines it's time to add the .then() handler to the microqueue for eventual execution?

const myPromise = new Promise(function(resolve, reject) {
    setTimeout(function() {
        resolve('Resolved promise: ');
    }, 2000);
});

myPromise.then((resolvedValue) => {
    console.log(resolvedValue + 'The .then() handler is now running');
});

// Expected Output (after ~2 seconds): "Resolved promise: The .then() handler is now running"

javascript

Answer 1

Answer №1

Answer

When you call the function resolve, it triggers a change in the promise's state, allowing JavaScript to detect that the state has been altered by your action of calling resolve.

The attached callback function with .then() is then scheduled to be executed as a microtask.

Deciphering your code

You create a new Promise and include a callback function that is immediately executed. This callback schedules a task to run after 2000ms, which will ultimately resolve the promise.

After setting up the promise, you attach a callback using .then() to the promise that will only be executed once the promise is fulfilled.

With the synchronous code completing its execution, the event loop can proceed to execute the next task. This may be the task scheduled after 2000ms, leading to the fulfillment of the promise (changing its state to "fulfilled").

Upon completion of the task to resolve the promise, a microtask is queued to run immediately afterward: This microtask will execute the .then() callback, resulting in the logging of your desired string.

Answer 2

Answer

When you call the function resolve, it triggers a change in the promise's state, allowing JavaScript to detect that the state has been altered by your action of calling resolve.

The attached callback function with .then() is then scheduled to be executed as a microtask.

Deciphering your code

You create a new Promise and include a callback function that is immediately executed. This callback schedules a task to run after 2000ms, which will ultimately resolve the promise.

After setting up the promise, you attach a callback using .then() to the promise that will only be executed once the promise is fulfilled.

With the synchronous code completing its execution, the event loop can proceed to execute the next task. This may be the task scheduled after 2000ms, leading to the fulfillment of the promise (changing its state to "fulfilled").

Upon completion of the task to resolve the promise, a microtask is queued to run immediately afterward: This microtask will execute the .then() callback, resulting in the logging of your desired string.

The method by which JavaScript identifies when a Promise has resolved or rejected

Answer №1

Answer

Deciphering your code

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