How JavaScript code gets executed ?

JavaScript Engine:

At the heart of JavaScript execution lies the JavaScript engine, the software component responsible for interpreting and executing JavaScript code. Major web browsers such as Google Chrome, Mozilla Firefox, and Microsoft Edge each have their own JavaScript engines – V8, SpiderMonkey, and Chakra respectively.

Let's understand the whole process of how JavaScript code is executed:

1. Parsing and Environment Setup:

   • JavaScript engine reads the script and sets up the global execution context.

   • Variables and functions are declared, but not initialized, during this phase.

2. Creation Phase:

   • Engine establishes the scope chain and allocates memory for variables and functions.

   • Initializes this keyword and outer environment references.

3. Execution Phase:

   • Code is executed line by line.

   • Function calls create new execution contexts with their own local scope.

   • Variables are assigned values as code is executed.

4. Function Execution Context:

   • Each function call creates a new execution context, placed on top of the call stack.

   • Local scope holds variables and functions declared within the function.

5. Completion and Cleanup:

   • Function execution contexts are added and removed from the call stack.

   • Empty call stack indicates completion of script execution.

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What are the ways to make the code Asynchronous?

There are several ways to make code asynchronous in JavaScript, allowing tasks to be executed concurrently without blocking the main thread. Here are some common techniques:

1. Callbacks:

• Callbacks are functions passed as arguments to other functions and executed once the asynchronous operation completes. They're commonly used in older JavaScript code and are still prevalent in many libraries and frameworks.

1. Promises:

• Promises provide a more structured way to handle asynchronous operations in JavaScript. They represent a value that may be available now, in the future, or never. Promises allow chaining of asynchronous operations and make error handling more straightforward.

1. Async/Await:

• Introduced in ES2017, async/await is a modern syntax for handling asynchronous code. It allows writing asynchronous code in a synchronous style, making it more readable and easier to understand. The async keyword is used to define a function that returns a promise, and the await keyword is used to pause the execution of the function until a promise is resolved.

1. Event Emitters:

• Event emitters provide a mechanism for asynchronous communication between different parts of an application. They allow registering event listeners for specific events and emitting events to trigger those listeners asynchronously.

1. Web Workers:

• Web Workers are a browser feature that allows running JavaScript code in background threads separate from the main execution thread. They're commonly used for CPU-intensive tasks to prevent blocking the main thread and maintain a responsive user interface.

1. SetTimeout and SetInterval:

• The setTimeout and setInterval functions are built-in JavaScript functions that allow scheduling code to run asynchronously after a specified delay or at regular intervals, respectively.

These are some of the main techniques for making code asynchronous in JavaScript. Each approach has its advantages and use cases, and the choice often depends on the specific requirements of the application and personal preference.

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What is event loop ?

• The event loop is a fundamental concept in JavaScript's concurrency model. It's responsible for handling asynchronous operations and ensuring that JavaScript remains single-threaded and non-blocking. Essentially, the event loop continuously checks the call stack and the message queue, moving tasks from the latter to the former when the call stack is empty.

Now let's understand how the event loop works.

1. Call Stack:

   • The call stack tracks the execution context of currently running functions in JavaScript, operating in a last-in, first-out (LIFO) manner.

2. Task Queue (Message Queue):

   • Asynchronous tasks are queued in the task queue after completion, awaiting execution when the call stack is empty.

3. Event Loop:

   • The event loop continuously monitors the call stack and task queue, moving tasks from the latter to the former when the call stack is clear, ensuring non-blocking execution.

4. Execution:

   • When a task is moved from the task queue to the call stack, its associated callback function is executed.

5. Continuation:

   • Upon completion, the execution context of a task is removed from the call stack, and the event loop continues to monitor for new tasks or events.

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What is callback ?

• A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

• In JavaScript programming, callbacks are pivotal for managing asynchronous tasks and events. They offer a way to execute code either synchronously or asynchronously, depending on the nature of the operation. Mastering both enables the creation of responsive and efficient JavaScript applications.

•   //example.
    function fetchData(callback) {
      const data = { id: 1, name: 'John Doe' };
      callback(data);
    }
    // Callback function to handle fetched data
    function handleData(data) {
      console.log('Received data:', data);
    }
  
    fetchData(handleData);
  

  This snippet demonstrates the use of a callback function in JavaScript. The fetchData function simulates fetching data and then calls the provided callback function (handleData) with the fetched data. In this example, handleData simply logs the received data to the console. Finally, fetchData is called with handleData as the callback function, indicating that handleData should be invoked once the data is fetched.

1. Synchronous Callbacks:

• Synchronous callbacks are executed immediately after the invocation of the outer function, without any intervening asynchronous tasks. They resemble a precise, step-by-step process, much like following a recipe meticulously.

• For instance, if you're baking a cake and setting a timer for 30 minutes before preheating the oven, it's akin to synchronous behavior. Each step follows the previous one directly, without waiting for external factors.

1. Asynchronous Callbacks:

• On the flip side, asynchronous callbacks are called after an asynchronous operation has completed. These operations typically involve tasks that take time to finish, such as fetching data from a server or waiting for user input.

• An asynchronous callback is like planning a party. You send out invitations and don't expect responses immediately. Instead, you carry on with other tasks until replies trickle in over time.

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What are web browser APIs?

• Web browser APIs (Application Programming Interfaces) are a set of tools and protocols provided by web browsers to enable developers to interact with various functionalities of the browser and the underlying operating system. These APIs expose a wide range of capabilities that developers can leverage to create dynamic, interactive, and feature-rich web applications.

Some common categories of web browser APIs.

• • DOM (Document Object Model) API: This API allows developers to manipulate the structure and content of web pages dynamically. It provides methods and properties to access, create, modify, and delete HTML elements and their attributes.

    • Web APIs: These APIs provide access to various web-related features such as fetching resources over the network (e.g., XMLHttpRequest, fetch API), interacting with servers (e.g., WebSocket API), and storing data locally (e.g., Web Storage API, IndexedDB API).

    • Geo location API: This API allows web applications to access the user's geographic location. It provides methods for retrieving the user's current location using GPS or other location information sources.

    • Device APIs: These APIs enable web applications to interact with device hardware and sensors. Examples include the Device Orientation API for accessing accelerometer and gyroscope data, the Battery Status API for checking the device's battery level, and the MediaDevices API for accessing cameras and microphones.

    • Storage APIs: Web browsers provide APIs for storing data locally on the user's device. Examples include the localStorage and sessionStorage APIs for storing key-value pairs in the browser's storage, and the IndexedDB API for storing structured data in a more powerful database-like manner.

• These are just a few examples of the many web browser APIs available to developers. By leveraging these APIs, developers can create web applications with rich features and functionalities that enhance the user experience and extend the capabilities of the web platform.