The setTimeout function is used to delay the execution of a piece of code for a specified amount of time. It is commonly used in JavaScript.

Here is an example of how to use setTimeout:

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// Define a function that will be executed after a delay
function greeting() {
  console.log("Hello, world!");
}

// Use setTimeout to delay the execution of the function by 3 seconds (3000 milliseconds)
setTimeout(greeting, 3000);

In this example, the greeting function will be executed after a delay of 3 seconds. The setTimeout function takes two arguments: the function to be executed, and the delay in milliseconds.

You can also pass additional arguments to the function by providing them as parameters after the delay. For example:

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function greet(name) {
  console.log("Hello, " + name + "!");
}

setTimeout(greet, 2000, "Alice");

In this case, the greet function will be executed after a delay of 2 seconds, with the argument "Alice". The output will be "Hello, Alice!".

Remember, setTimeout is asynchronous, meaning that it doesn't stop the execution of the rest of your code. If you need to perform tasks after the delay, you can use a callback function or a promise.

A promise in JavaScript is an object that represents the eventual completion or failure of an asynchronous operation. It is used for handling asynchronous operations, such as making API calls or reading files, in a more organized and readable way.

Promises have three states:

• Pending: The initial state. The promise is still being resolved or rejected.
• Fulfilled: The asynchronous operation has been completed successfully, and the promise is resolved. It returns a value.
• Rejected: The asynchronous operation has encountered an error, and the promise is rejected. It returns an error.

Promises have two main methods:

• then(): This method is called when the promise is fulfilled. It takes two optional callbacks, one for success and one for failure. The success callback receives the resolved value, and the failure callback receives the rejected error.
• catch(): This method is called when the promise is rejected. It takes a callback function that receives the error.

Here's an example of using a promise to fetch data from an API:

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fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => {
    // Do something with the fetched data
    console.log(data);
  })
  .catch(error => {
    // Handle any errors that occurred during the fetch
    console.error(error);
  });

In this example, the fetch() function returns a promise. We can use the then() method to handle the response and parse it as JSON. If there are any errors during the fetch, the catch() method can handle them.

AJAX stands for Asynchronous JavaScript and XML. It is a technique used in web development to send and receive data asynchronously from a web server without having to reload the entire webpage. AJAX allows for updating parts of a webpage without refreshing the entire page. It is commonly used to fetch data from a server, submit form data, and update dynamic content on a webpage.

Here is an example of using AJAX to fetch data from a server using JavaScript:

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// Create an XMLHttpRequest object
var xhr = new XMLHttpRequest();

// Set up the request
xhr.open('GET', 'https://api.example.com/data', true);

// Send the request
xhr.send();

// Listen for the response
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    // Process the response data
    var response = JSON.parse(xhr.responseText);
    console.log(response);
  }
};

AJAX stands for Asynchronous JavaScript And XML. It is a web development technique that allows you to send and receive data from a web server without needing to reload the entire web page. With AJAX, you can update specific parts of a web page asynchronously, meaning the user can continue interacting with the page while data is being exchanged with the server in the background. This helps create a more seamless and dynamic user experience on websites. AJAX uses a combination of JavaScript and XML (although nowadays JSON is more commonly used) to handle the data transfer. Overall, AJAX provides a way for web pages to be more interactive and responsive.

A promise is a feature introduced in ES6 (ECMAScript 2015), which allows us to work with asynchronous events in a more synchronous way. It's a way to handle the results of an operation that may take some time to complete.

Promises are objects in JavaScript that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise has three states:

• pending: The initial state, neither fulfilled nor rejected.
• fulfilled: Meaning that the operation completed successfully, and the promise has a resulting value (also known as resolve).
• rejected: Meaning that the operation failed, and the promise has a reason for that failure (also known as rejected).

A promise is always created with the new Promise() constructor. The constructor takes a function as a parameter, which is called the executor function. This executor function takes two parameters which both are functions resolve and reject. The resolve function is called, when the operation is completed, and the promise has a value to return. The reject function is called, when the operation fails, and the promise has a reason for that failure.

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const promise = new Promise((resolve, reject) => {
  const computation = Math.random() * 100;
  setTimeout(() => {
    if (computation < 50) {
      resolve(computation);
    } else {
      reject(`Computation is too high: ${computation}`);
    }
  }, 1000);
});

promise.then(result => {
  console.log(`Computation result: ${result}`);
}).catch(error => {
  console.error(`Error occurred: ${error}`);
});

In the example above, we created a promise, which resolves after a random time delay and returns the computed number unless the result is too high. If the computation is lower than 50, the promise resolves and logs the Computation result. However, when the computation is higher than 50, the promise is rejected and logs the Error occurred.

Promises can also be chained. Multiple then() function can be called, connecting them with each other. Each then() function receives the result of the previous one.

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const endpoint = "https://jsonplaceholder.typicode.com/todos/1";
fetch(endpoint)
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.log(error));

In the code above, we first call fetch() to load data from an API, then we parse the response using .json(), and finally, we log the resulting JSON object. If at any point during the chain an error occurs, we can catch it using .catch() function which will go to the error block and log the exception.

Using Promises can make our code more readable especially when dealing with complex asynchronous operations.

Asynchronous JavaScript refers to the ability of JavaScript to perform tasks without having to wait for other tasks that are being executed. In other words, it allows multiple things to happen at once.

In synchronous code, task B cannot begin before task A has finished. So, if Task A is a function that takes a long time to execute, Task B will be blocked from running until Task A has completed. With asynchronous code, however, Task B can still run while Task A is executing. It is important to note that in most cases tasks do not actually run in parallel, they just appear to.

Asynchronous code is commonly used when working with time-consuming tasks such as fetching data from an API or reading files. A common approach to handle asynchronous code is to use callbacks, promises, or async/await.

Callbacks are functions that are passed as arguments to another function and are executed when the parent function has completed its task.

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//Callback example

function taskA(callback) {       
  setTimeout(function () {    
    console.log("Task A has completed.");       
    callback();                 
  }, 3000)                   
}

function taskB() {               
  console.log("Task B has completed.");     
}

taskA(taskB);                       

Promises are used to handle the results of an asynchronous operation. They allow you to chain together multiple asynchronous calls with then() statements.

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//Promise example

function taskA() {          
  return new Promise(function (resolve, reject) {      
    setTimeout(function () {               
      console.log("Task A has completed.");         
      resolve();                        
    }, 3000)                         
  })
}

function taskB() {                  
  console.log("Task B has completed.");     
}

taskA().then(taskB);                     

async/await is a newer syntax that provides a cleaner way to write asynchronous code. Async functions always return a promise and await can be used to wait for the result of an asynchronous operation.

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//async/await example

async function taskA() {      
  await new Promise(function (resolve, reject) {     
    setTimeout(function () {                  
      console.log("Task A has completed.");         
      resolve();                           
    }, 3000)                        
  })
}

function taskB() {                                
  console.log("Task B has completed.");      
}

taskA().then(taskB);                              

It is important to use asynchronous code in the right situations as it may not always be beneficial. However, when used correctly, it can greatly improve the performance of your code.

In JavaScript, a callback function is a function that is passed as an argument to another function and is executed after some event occurs or a certain task is completed. Callback functions are used to make code more modular and flexible, and are commonly used for asynchronous operations such as fetching data or handling user events. Here is an example in JavaScript:

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function fetchData(callback) {
  // code to fetch data from a server
  let data = [1, 2, 3, 4, 5];
  // invoke the callback function with the fetched data as an argument
  callback(data);
}

// define a callback function to process the fetched data
function processData(data) {
  // code to process the data
  console.log(data);
}

// pass the processData function as a callback to the fetchData function
fetchData(processData);

In this example, the fetchData function takes a callback function as an argument and calls it with the fetched data. The processData function is defined separately and passed as a callback to fetchData. This allows for greater flexibility and reusability of the code.

Ajax stands for "Asynchronous JavaScript And XML." It's a set of web development techniques that allows web applications to send and receive data from a server without having to reload the entire webpage. It can be used for building dynamic and responsive web pages where data is loaded asynchronously in the background, allowing for a smoother and faster user experience.

Here's an example of how to use Ajax with JavaScript:

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// Creating a new XMLHttpRequest object
let xhttp = new XMLHttpRequest();

// Event listener to handle the response from the server
xhttp.onreadystatechange = function() {
  if (this.readyState == 4 && this.status == 200) {
    // Code to handle the response from the server
    console.log(this.responseText);
  }
};

// Setting up the request to the server using the open() and send() methods
xhttp.open("GET", "https://example.com/data", true);
xhttp.send();

In the example above, we create a new XMLHttpRequest object and set up an event listener to handle the response from the server when it's received. We then set up the request using the open() and send() methods, specifying the HTTP method and the URL we want to send the request to. When the response is received, the code inside the event listener is executed, allowing us to access the data sent back from the server.

In TypeScript, a Promise<number> is a type definition that represents a promise that will eventually return a number. A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and is commonly used to handle asynchronous operations like network requests, database queries, and other operations that may take time.

Here's an example of how you can define and use a Promise<number> in TypeScript:

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function fetchNumber(): Promise<number> {
  return new Promise<number>((resolve, reject) => {
    // some asynchronous operation, e.g., fetch a number from server
    const randomNum = Math.floor(Math.random() * 10);
    if (randomNum >= 5) {
      resolve(randomNum); // operation succeeded, return the number
    } else {
      reject("Failed to fetch a number."); // operation failed, return an error
    }
  });
}

// usage:
fetchNumber()
  .then((num) => {
    console.log(`The number is: ${num}`);
  })
  .catch((error) => {
    console.error(`An error occurred: ${error}`);
  });

In this example, fetchNumber() is a function that returns a Promise<number>. It creates a new promise and defines the code to execute when the operation succeeds (resolve) or fails (reject). Then, we use fetchNumber() by calling it and chaining .then() and .catch() methods to handle the eventual completion of the promise. If the operation succeeds, the .then() method is called with the number value as its argument; otherwise, the .catch() method is called with the error message as its argument.

Node.js is an open-source, cross-platform runtime environment that allows developers to run JavaScript code outside of a web browser.

Some of the features of Node.js include:

• Asynchronous and event-driven: It uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, making it ideal for building scalable network applications.
• Fast: Being built on top of Google's V8 JavaScript engine, Node.js is very fast in code execution.
• Single-threaded: Node.js uses a single-threaded model with event looping which enables it to handle a large amount of incoming connections, making it highly scalable.
• Large module library: Node.js has a large library of modules written by other developers that can be easily integrated into your project.

Node.js works by running JavaScript code outside of the web browser environment, instead using the V8 JavaScript engine to execute the code. It reads the JavaScript file, executes the code, and returns the result.

Here's an example of a basic HTTP server in Node.js:

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const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello, world!');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

This code creates an HTTP server that listens for incoming requests on port 3000. When a request is received, it responds with a plain text message of "Hello, world!".

You can learn more about Node.js on their official website: https://nodejs.org/en/about/

Promises are used in JavaScript to handle asynchronous operations. They offer a way to handle errors and outcomes of asynchronous functions, such as those used in AJAX requests or multi-step tasks. Promises provide an object that describes the status of a task and the ability to chain methods.

For example, you can use a promise chain to properly handle a series of requests:

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const myTask = () => {
  return new Promise((resolve, reject) => {
    // perform some asynchronous operation
    setTimeout(() => {
      // operation finished
      if (conditionMet) {
        resolve("The task succeeded")
      } else {
        reject("The task failed")
      }
    }, 1000);
  });
}

myTask()
  .then(res => console.log(res))
  .catch(err => console.log(err))

In the example above, the asynchronous operation is the setTimeout() function and the promise chain will console.log() the result or the error depending on the condition of the operation.

AJAX (Asynchronous JavaScript and XML) is a technique used to create asynchronous web applications. It allows web applications to send data to and from a server asynchronously without affecting the display of the page. It uses a combination of HTML, JavaScript and XML to update parts of a web page without refreshing the whole page.

The difference between AJAX and JavaScript is that AJAX is used for asynchronous communication, while JavaScript is used for building interactive webpages. JavaScript is a scripting language that runs on the client-side and can be used for more than just AJAX. It is a versatile language capable of handling not only AJAX operations, but also client-side tasks such as form validation, interactivity and animations.

Javascript can appear difficult at first due to its asynchronous nature, the need for an understanding of the Document Object Model (DOM), and its use of functions. However, these features make Javascript incredibly powerful and an incredibly popular programming language. With practice and a good understanding of the fundamentals of programming, it's possible to become quite proficient in Javascript. For more resources on learning Javascript, check out the Mozilla Developer Network.

Common node.js interview questions include:

• What is Node.js used for?
• What is the difference between Node.js and JavaScript?
• What is asynchronous programming?
• What are the pros and cons of using an asynchronous programming paradigm?
• What is an event loop?
• What libraries and frameworks have you used in your projects?
• How do you debug a Node.js application?
• How do you unit test a Node.js application?
• How would you secure a Node.js application?
• Describe the most complex project you’ve built with Node.js.
• How do streams work in Node.js?
• What is the responsibility of the incubator in the Node.js process?
• What is callback hell and how can it be avoided?
• What do you need to keep in mind when scaling a Node.js application?

See this Stack Overflow answer for an in-depth look into Node.js interview questions.