javascript practice question and answer each topic 10 qustion

Variables and Data Types

Question: How do you declare a variable in JavaScript?
Answer:

   let x;

Question: What is the difference between let and const?
Answer:

   // `let` allows you to reassign values to a variable.
   let a = 10;
   a = 20; // No error

   // `const` does not allow reassignment.
   const b = 10;
   // b = 20; // Error

Question: What are the primitive data types in JavaScript?
Answer:

   // Number, String, Boolean, Undefined, Null, Symbol, BigInt

Question: How do you check the type of a variable?
Answer:

   console.log(typeof 42); // "number"

Question: What will console.log(typeof null) output?
Answer:

   console.log(typeof null); // "object"

Question: How do you convert a string to a number in JavaScript?
Answer:

   let num = Number("123"); // 123

Question: How do you declare a constant in JavaScript?
Answer:

   const PI = 3.14;

Question: What is undefined in JavaScript?
Answer:

   // A variable that has been declared but not assigned a value is `undefined`.
   let a;
   console.log(a); // undefined

Question: What is the difference between null and undefined?
Answer:

   // `undefined` means a variable has been declared but not assigned a value.
   // `null` is an assignment value that represents no value or no object.

Question: How do you concatenate two strings in JavaScript?
Answer:
javascript let str1 = "Hello, "; let str2 = "World!"; let result = str1 + str2; // "Hello, World!"

Operators and Expressions

Question: What is the difference between == and ===?
Answer:

   // `==` checks for equality with type coercion.
   console.log(5 == '5'); // true

   // `===` checks for equality without type coercion.
   console.log(5 === '5'); // false

Question: What will console.log(10 % 3) output?
Answer:

   console.log(10 % 3); // 1

Question: How do you increment and decrement a variable?
Answer:

   let x = 10;
   x++; // 11
   x--; // 10

Question: What is the result of true && false?
Answer:

   console.log(true && false); // false

Question: What is the result of true || false?
Answer:

   console.log(true || false); // true

Question: How do you use the ternary operator in JavaScript?
Answer:

   let age = 18;
   let canVote = age >= 18 ? 'Yes' : 'No';
   console.log(canVote); // "Yes"

Question: What does !true evaluate to?
Answer:

   console.log(!true); // false

Question: How do you add two numbers in JavaScript?
Answer:

   let sum = 5 + 10; // 15

Question: How do you multiply two numbers in JavaScript?
Answer:

   let product = 5 * 10; // 50

Question: What is the result of 2 ** 3 in JavaScript?
Answer:
javascript console.log(2 ** 3); // 8

Functions and Scope

Question: How do you declare a function in JavaScript?
Answer:

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

Question: What is a function expression?
Answer:

   const greet = function() {
     console.log("Hello!");
   };

Question: What is the difference between function declarations and function expressions?
Answer:

   // Function declarations are hoisted to the top of their scope.
   function greet() {
     console.log("Hello!");
   }

   // Function expressions are not hoisted.
   const greet = function() {
     console.log("Hello!");
   };

Question: How do you create an immediately invoked function expression (IIFE)?
Answer:

   (function() {
     console.log("This is an IIFE");
   })();

Question: What is the scope of a variable declared with var?
Answer:

   // Variables declared with `var` are function-scoped.
   function test() {
     var x = 10;
     if (true) {
       var y = 20;
     }
     console.log(y); // 20
   }

Question: What is the scope of a variable declared with let or const?
Answer:

   // Variables declared with `let` and `const` are block-scoped.
   function test() {
     let x = 10;
     if (true) {
       let y = 20;
     }
     // console.log(y); // Error: y is not defined
   }

Question: How do you return a value from a function?
Answer:

   function add(a, b) {
     return a + b;
   }
   console.log(add(2, 3)); // 5

Question: How do you pass arguments to a function?
Answer:

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

Question: What is a closure in JavaScript?
Answer:

   // A closure is a function that has access to its own scope, the outer function's scope, and the global scope.
   function outer() {
     let outerVar = "Outer";
     function inner() {
       console.log(outerVar);
     }
     return inner;
   }
   let innerFunc = outer();
   innerFunc(); // "Outer"

Question: What is a higher-order function?
Answer:
javascript // A higher-order function is a function that takes another function as an argument or returns a function. function higherOrder(func) { func(); } higherOrder(function() { console.log("Hello from the higher-order function"); });

Arrays and Objects

Question: How do you create an array in JavaScript?
Answer:

   let arr = [1, 2, 3];

Question: How do you access the first element of an array?
Answer:

   let arr = [1, 2, 3];
   console.log(arr[0]); // 1

Question: How do you add an element to the end of an array?
Answer:

   let arr = [1, 2, 3];
   arr.push(4);
   console.log(arr); // [1, 2, 3, 4]

Question: How do you remove the last element of an array?
Answer:

   let arr = [1, 2, 3];
   arr.pop();
   console.log(arr); // [1, 2, 3]

Question: How do you create an object in JavaScript?
Answer:

   let obj = {
     name: "Alice",
     age: 25
   };

Question: How do you access the value of a property in an object?
Answer:

   let obj = {
     name: "Alice",
     age: 25
   };
   console.log(obj.name); // "Alice"

Question: How do you add a property to an object?
Answer:

   let obj = {
     name: "Alice",
     age: 25


   };
   obj.job = "Developer";
   console.log(obj); // { name: "Alice", age: 25, job: "Developer" }

Question: How do you remove a property from an object?
Answer:

   let obj = {
     name: "Alice",
     age: 25
   };
   delete obj.age;
   console.log(obj); // { name: "Alice" }

Question: What method can be used to find the length of an array?
Answer:

   let arr = [1, 2, 3];
   console.log(arr.length); // 3

Question: How do you iterate over an array using a for loop?
Answer:
javascript let arr = [1, 2, 3]; for (let i = 0; i < arr.length; i++) { console.log(arr[i]); }

DOM Manipulation

Question: How do you select an element by its ID?
Answer:

   let element = document.getElementById('myElement');

Question: How do you select elements by their class name?
Answer:

   let elements = document.getElementsByClassName('myClass');

Question: How do you select elements using a CSS selector?
Answer:

   let elements = document.querySelectorAll('.myClass');

Question: How do you change the text content of an element?
Answer:

   let element = document.getElementById('myElement');
   element.textContent = 'New Text';

Question: How do you change the HTML content of an element?
Answer:

   let element = document.getElementById('myElement');
   element.innerHTML = '<strong>New HTML</strong>';

Question: How do you add a class to an element?
Answer:

   let element = document.getElementById('myElement');
   element.classList.add('newClass');

Question: How do you remove a class from an element?
Answer:

   let element = document.getElementById('myElement');
   element.classList.remove('oldClass');

Question: How do you add an event listener to an element?
Answer:

   let button = document.getElementById('myButton');
   button.addEventListener('click', function() {
     console.log('Button clicked');
   });

Question: How do you prevent the default action of an event?
Answer:

   let link = document.getElementById('myLink');
   link.addEventListener('click', function(event) {
     event.preventDefault();
   });

Question: How do you stop event propagation?
Answer:
javascript let button = document.getElementById('myButton'); button.addEventListener('click', function(event) { event.stopPropagation(); });

Promises and Asynchronous JavaScript

Question: What is a promise in JavaScript?
Answer:

   // A promise is an object that represents the eventual completion (or failure) of an asynchronous operation.

Question: How do you create a new promise?
Answer:

   let promise = new Promise((resolve, reject) => {
     // Asynchronous operation
     if (success) {
       resolve(result);
     } else {
       reject(error);
     }
   });

Question: How do you handle a fulfilled promise?
Answer:

   promise.then(result => {
     console.log(result);
   });

Question: How do you handle a rejected promise?
Answer:

   promise.catch(error => {
     console.log(error);
   });

Question: How do you execute code after a promise is settled, regardless of its outcome?
Answer:

   promise.finally(() => {
     console.log('Promise settled');
   });

Question: What does the async keyword do?
Answer:

   // The `async` keyword makes a function return a promise.
   async function fetchData() {
     return "data";
   }

Question: What does the await keyword do?
Answer:

   // The `await` keyword pauses the execution of an async function until the promise is resolved.
   async function fetchData() {
     let data = await fetch('url');
     console.log(data);
   }

Question: How do you handle multiple promises in parallel?
Answer:

   Promise.all([promise1, promise2]).then(results => {
     console.log(results);
   });

Question: What does Promise.race() do?
Answer:

   // `Promise.race()` returns a promise that resolves or rejects as soon as one of the promises in the array resolves or rejects.
   Promise.race([promise1, promise2]).then(result => {
     console.log(result);
   });

Question: How do you create a promise that resolves after a certain amount of time?
Answer:
javascript function delay(ms) { return new Promise(resolve => setTimeout(resolve, ms)); } delay(1000).then(() => console.log('Resolved after 1 second'));

These questions and answers should provide a comprehensive review of fundamental and intermediate JavaScript concepts across various topics.

javascript practice question and answer each topic 10 qustion

Variables and Data Types

Operators and Expressions

Functions and Scope

Arrays and Objects

DOM Manipulation

Promises and Asynchronous JavaScript

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