Promises and code flow.

Promises are a built-in feature in JavaScript that helps to manage asynchronous operations. They are objects that represent the eventual completion or failure of an asynchronous operation and its resulting value. Promises provide a more organized way of handling callbacks by allowing us to chain them and manage them more efficiently.

The basic structure of a Promise is as follows:

const myPromise = new Promise((resolve, reject) => {
  // some asynchronous operation
  if (/* operation was successful */) {
    resolve(result);
  } else {
    reject(error);
  }
});

Here, myPromise is an instance of the Promise class. It takes a callback function with two parameters: resolve and reject. resolve is called when the operation is successful, and reject is called when the operation fails. The result is the value returned when the operation is successful, and error is the value returned when the operation fails.

Code Flow through Call Stack, Web API, and Microtask Queue

Now that we understand what Promises are, let's see how they work through the call stack, web API, and microtask queue.

Call Stack

The call stack is a data structure in JavaScript that tracks the execution of function calls. When a function is called, it is added to the top of the call stack. When the function completes its execution, it is removed from the stack.

Let's see an example:

function foo() {
  console.log('Hello, World!');
}

function bar() {
  foo();
}

bar();

When bar() is called, it is added to the call stack. Then bar() calls foo(), which is added to the top of the stack. When foo() completes its execution, it is removed from the stack. Then bar() completes its execution and is removed from the stack.

Web API

Web APIs are built-in browser APIs that allow us to interact with the browser environment. They include features such as timers, network requests, and the DOM.

When an asynchronous operation is called, such as a setTimeout() function, it is passed to the web API for execution. The function is not added to the call stack because it is executed asynchronously.

Let's see an example:

console.log('Before');

setTimeout(() => {
  console.log('After');
}, 2000);

console.log('End');

When this code is executed, console.log('Before') is added to the call stack, and it is executed. Then setTimeout() is called, and it is passed to the web API for execution. Since setTimeout() is an asynchronous function, it is not added to the call stack. Instead, it is passed to the web API for execution, and a timer is set for two seconds.

While the timer is running, the execution continues with console.log('End'), which is added to the call stack and executed. Then, after two seconds, the callback function passed to setTimeout() is added to the task queue.

Microtask Queue

The microtask queue is a queue that holds callback functions for Promises. When a Promise is resolved or rejected, its callback function is added to the microtask queue. The microtask queue has a higher priority than the task queue, which means that its functions are executed before

Here's an example of a Promise and its code flow through the call stack, web API, and microtask queue:

console.log('Before Promise');

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved!');
  }, 2000);
});

console.log('After Promise');

myPromise
  .then((result) => {
    console.log(result);
  })
  .catch((error) => {
    console.log(error);
  });

console.log('End of script');

In this example, we create a Promise that resolves after a 2-second delay using the setTimeout() function. We then add console.log() statements to track the flow of the code.

Here's the code flow:

console.log('Before Promise') is added to the call stack and executed.
The Promise is created and console.log('After Promise') is added to the call stack and executed.
The Promise is resolved after 2 seconds and its callback function is added to the microtask queue.
console.log('End of script') is added to the call stack and executed.
The Promise's callback function is added to the call stack from the microtask queue.
The then() method is called on the Promise, and its callback function is added to the microtask queue.
The Promise's callback function is removed from the stack, and the then() callback function is added to the stack.
The then() callback function is executed with the result passed from the Promise's callback function.
The catch() method is not called because the Promise was resolved successfully.
The then() callback function is removed from the stack.

what is promise's call back function here?

In the example code I provided earlier, the Promise's callback function is the function that is passed as an argument to the Promise constructor. This function takes two parameters, resolve and reject, which are also functions.

Here's the relevant portion of the code again:

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved!');
  }, 2000);
});

In this example, the Promise's callback function is:

(resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved!');
  }, 2000);
}

This function uses the setTimeout() function to simulate an asynchronous operation that takes 2 seconds to complete. Once the operation is complete, the resolve function is called with the result 'Promise resolved!'.

When the Promise is resolved, its callback function is passed to the then() method as a parameter, like this:

myPromise
  .then((result) => {
    console.log(result);
  })
  .catch((error) => {
    console.log(error);
  });

Here, the then() method takes a callback function as its parameter, which is executed when the Promise is resolved. This function takes the result passed to the resolve() function as its parameter, and logs it to the console.

So, in summary, the Promise's callback function is the function passed to the Promise constructor, which is executed asynchronously and resolves the Promise. Once the Promise is resolved, its callback function is passed to the then() method as a parameter, where it is executed with the resolved result.

Extras

console.log(fetch(url)) vs console.log(fetch(url).then(cb => cb)

console.log(fetch(url))console.log(fetch(url).then(cb => cb)

The code console.log(fetch(url).then(cb => cb)) performs a fetch request to the URL specified by the url variable and then logs the resolved value of the Promise returned by the then() method to the console.

The fetch function is a built-in method in JavaScript that is used to make HTTP requests. It takes a URL as an argument and returns a Promise that resolves to a Response object representing the response to the request.

The then() method is called on the Promise returned by fetch(). It takes a callback function as an argument that will be invoked when the Promise is resolved with the Response object. In this case, the callback simply returns the Response object as is, without doing anything else.

When the Promise resolves, the resolved value (i.e., the Response object) is passed as an argument to the callback function provided to then(). Since the callback simply returns the Response object as is, the resolved value of the Promise is the same as the Response object.

The console.log() function is used to log the resolved value of the Promise (i.e., the Response object) to the console.

CODEFLOW-

When the code console.log(fetch(url).then(cb => cb)) is executed, it is added to the call stack.
The fetch() function is called and returns a Promise, which is pushed to the call stack.
Since the Promise returned by fetch() is an asynchronous operation, the code continues to run and the Promise is sent to the Web API environment to be handled asynchronously.
The then() method is called on the Promise returned by fetch(), and a callback function is passed as an argument. This also returns a Promise, which is pushed to the call stack.
The callback function is sent to the microtask queue (not the Web API environment) to be executed after the current synchronous code has finished executing.
The Promise returned by the then() method is returned and resolved with the Response object once the asynchronous fetch() operation completes.
Once the current synchronous code has finished executing, the callback function is dequeued from the microtask queue and pushed to the call stack to be executed.
The callback function simply returns the Response object as is.
The resolved value of the Promise (i.e., the Response object) is logged to the console using console.log().
The call stack is cleared, and the program continues to run any remaining code or waits for further user input.

console.log(fetch(url))

The code console.log(fetch(url)) performs a fetch request to the URL specified by the url variable and then logs the Promise returned by the fetch() method to the console.

Here's how the code flow would happen through the call stack, web APIs, and microtask queue:

When the code console.log(fetch(url)) is executed, it is added to the call stack.
The fetch() function is called and returns a Promise, which is pushed to the call stack.
Since the Promise returned by fetch() is an asynchronous operation, the code continues to run and the Promise is sent to the Web API environment to be handled asynchronously.
The Promise returned by fetch() is returned to the code, and its value is logged to the console using console.log().
The call stack is cleared, and the program continues to run any remaining code or waits for further user input.

Note that in this case, there is no then() method called on the Promise returned by fetch(), so there is no callback function to be sent to the microtask queue. The Promise itself is resolved in the Web API environment when the fetch() operation completes, and its resolved value (i.e., the Response object) is passed to the console.log() method.

-- The code console.log(fetch(url)) will log a Promise object to the console

--The code console.log(fetch(url).then(cb => console.log(cb))) will log a Promise object to the console, followed by logging the response object to the console when the Promise resolves.