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What is event delegation in JavaScript and how does it work?
What is the difference between the var, let, and const keywords in JavaScript?
What is the this keyword in JavaScript and how does it work?
What is the difference between null and undefined in JavaScript?
What is a closure in JavaScript and how is it used?
What is a callback function in JavaScript and how is it used?
What is a generator function in JavaScript and how is it used?
What is the difference between == and === in JavaScript?
What is a higher-order function in JavaScript and how is it used?
What is the difference between apply, call, and bind in JavaScript?

Advanced JavaScript Questions with answers, code examples and explanations

What is event delegation in JavaScript and how does it work?

Answer: Event delegation is a technique used to handle events efficiently by attaching a single event listener to a parent element, rather than multiple event listeners to child elements. When an event occurs on a child element, the event bubbles up through the DOM hierarchy to the parent element, which can then handle the event. This approach reduces the number of event listeners required and can improve performance.

Code example:

document.addEventListener(‘click’, function(event) {

  if (event.target.matches(‘.btn’)) {

    // Handle button click event

  }

});

Explanation: In this example, the click event is attached to the document object. When a click event occurs on any element within the document, the event is captured and the event.target property is checked to see if it matches the .btn selector. If it does, the button click event is handled.

What is the difference between the var, let, and const keywords in JavaScript?

Answer: var, let, and const are all used to declare variables in JavaScript, but they have different scoping rules and behaviors. var has function-level scope, meaning that a variable declared with var is accessible within the function in which it was declared, as well as any nested functions. let and const have block-level scope, meaning that a variable declared with let or const is only accessible within the block in which it was declared.

let is used to declare a variable that can be reassigned a new value. const is used to declare a variable that cannot be reassigned, but the value can be modified if it is a mutable data type.

Code example:

function exampleFunction() {

  var x = 1;

  if (true) {

    let y = 2;

    const z = [3];

    z.push(4);

    console.log(z); // Output: [3, 4]

  }

  console.log(x); // Output: 1

  console.log(y); // ReferenceError: y is not defined

  console.log(z); // ReferenceError: z is not defined

}

exampleFunction();

Explanation: In this example, var x is declared within the function and is accessible throughout the function, including the if block. let y and const z are declared within the if block and are only accessible within that block. z is an array, which is a mutable data type, so its contents can be modified even though it is declared with const. When the function is called, x is logged to the console, but attempting to log y or z outside of the if block results in a ReferenceError.

What is the this keyword in JavaScript and how does it work?

Answer: The this keyword in JavaScript refers to the current execution context, which can vary depending on how a function is called. When used within a function, this can refer to the global object (window in a browser), the object that the function is a method of, or a new object created with the new keyword.

Code example:

let person = {

  name: ‘John’,

  age: 30,

  sayHello: function() {

    console.log(`Hello, my name is ${this.name}`);

  }

}

person.sayHello(); // Output: Hello, my name is John

Explanation: In this example, this refers to the person object because the sayHello function is called as a method of that object using dot notation. Within the sayHello function, this.name refers to the name property of the person object. If the function were called without the person object as its context, this would refer to the global object instead.

What is the difference between null and undefined in JavaScript?

Answer: null and undefined are both special values in JavaScript used to represent the absence of a value. However, they have different meanings. undefined is the default value assigned to a variable that has been declared but not assigned a value, or to a function parameter that has not been passed a value. null is a value that can be assigned to a variable to represent the absence of any object value.

Code example:

let x;

console.log(x); // Output: undefined

let y = null;

console.log(y); // Output: null

Explanation: In this example, x is declared but not assigned a value, so its value is undefined. y is assigned the value null explicitly.

What is a closure in JavaScript and how is it used?

Answer: A closure is a function that has access to its parent function’s scope, even after the parent function has completed execution. Closures are created by returning an inner function from an outer function, and can be used to create private variables and functions, as well as implement callback functions and event handlers.

Code example:

function outer() {

  let count = 0;

  function inner() {

    count++;

    console.log(count);

  }

  return inner;

}

let counter = outer();

counter(); // Output: 1

counter(); // Output: 2

Explanation: In this example, the outer function returns the inner function, which has access to the count variable in the outer function’s scope. The counter variable is assigned the inner function, and can be called multiple times to increment and display the count variable.

What is a callback function in JavaScript and how is it used?

Answer: A callback function is a function that is passed as an argument to another function, and is executed after the main function completes its operation. Callback functions are commonly used in JavaScript to implement asynchronous operations, such as handling user input, making API requests, and processing large datasets.

Code example:

function doSomething(callback) {

  // Perform some operation

  callback();

}

function doSomethingElse() {

  console.log(‘Done!’);

}

doSomething(doSomethingElse);

Explanation: In this example, the doSomething function accepts a callback argument, which is called after the main function completes its operation. The doSomethingElse function is passed as a callback argument to doSomething, and is executed after doSomething completes its operation.

What is a generator function in JavaScript and how is it used?

Answer: A generator function is a special type of function that can pause its execution and resume it later, allowing for the generation of a sequence of values over time. Generator functions are created using the function* syntax and use the yield keyword to return a value and pause execution. They can be used to implement lazy evaluation, infinite sequences, and asynchronous programming.

Code example:

function* fibonacci() {

  let a = 0, b = 1;

  while (true) {

    yield a;

    [a, b] = [b, a + b];

  }

}

let gen = fibonacci();

console.log(gen.next().value); // Output: 0

console.log(gen.next().value); // Output: 1

console.log(gen.next().value); // Output: 1

console.log(gen.next().value); // Output: 2

Explanation: In this example, the fibonacci function is a generator function that yields each number in the Fibonacci sequence. The gen variable is assigned the generator object returned by the fibonacci function, and the next method is called to retrieve each value in sequence.

What is the difference between == and === in JavaScript?

Answer: == and === are both comparison operators in JavaScript, used to compare two values for equality. However, they have different rules for type coercion. == performs type coercion, meaning it will attempt to convert the operands to a common type before comparison. === does not perform type coercion, and will only return true if the operands are of the same type and value.

Code example:

console.log(5 == ‘5’); // Output: true

console.log(5 === ‘5’); // Output: false

Explanation: In this example, the == operator converts the string ‘5’ to the number 5 before comparison, and returns true. The === operator does not perform type coercion, and returns false because the operands are of different types.

What is a higher-order function in JavaScript and how is it used?

Answer: A higher-order function is a function that takes one or more functions as arguments, or returns a function as its result. Higher-order functions are commonly used in functional programming to create reusable and composable code.

Code example:

function double(arr) {

  return arr.map(function(x) {

    return x * 2;

  });

}

let numbers = [1, 2, 3, 4];

console.log(double(numbers)); // Output: [2, 4, 6, 8]

Explanation: In this example, the double function is a higher-order function that takes an array as an argument and returns a new array with each element doubled. The map method is a higher-order function that applies the provided function to each element of the array, and returns a new array with the transformed values.

What is the difference between apply, call, and bind in JavaScript?

Answer: apply, call, and bind are all methods used to call a function with a specific this value, as well as to pass arguments to the function. The main difference between them is how they handle the this value and arguments.

call and apply are similar, but apply takes an array of arguments, while call takes arguments directly. bind returns a new function with a bound this value and arguments, but does not immediately call the function.

Code example:

const person = {

  name: ‘John’,

  sayHi: function(greeting) {

    console.log(`${greeting}, my name is ${this.name}`);

  }

}

const person2 = {

  name: ‘Jane’

}

person.sayHi(‘Hello’); // Output: Hello, my name is John

person.sayHi.call(person2, ‘Hi’); // Output: Hi, my name is Jane

person.sayHi.apply(person2, [‘Hi’]); // Output: Hi, my name is Jane

const bound = person.sayHi.bind(person2, ‘Hey’);

bound(); // Output: Hey, my name is Jane

Explanation: In this example, the person object has a sayHi method that logs a greeting with the this.name value. call is used to call the sayHi method with a different this value, person2. apply is used to call the sayHi method with a different this value and an array of arguments. bind is used to create a new function with a bound this value and argument, which is then called with the bound function.