50 simple javascript questions

Here are 50 simple Javascript questions along with their answers and code examples:

• What is the output of console.log(5 + "5") in JavaScript?
  • This will output 55. Because one operand is a string, JavaScript will convert the other operand to a string before performing the concatenation.
• What is the output of console.log(5 - "5") in JavaScript?
  • This will output 0. JavaScript performs type coercion and converts the string "5" to the number 5, then does the subtraction.
• What is the output of console.log(5 == "5") in JavaScript?
  • This will output true. The double equals == operator performs type coercion if the types on both sides are different.
• What is the output of console.log(5 === "5") in JavaScript?
  • This will output false. The triple equals === operator does not perform type coercion.
• Write a function to reverse a string in JavaScript.

   function reverseString(str) {
     return str.split('').reverse().join('');
   }

• How do you declare a variable in JavaScript?

   var myVariable = "Hello";
   let myVariable = "Hello";
   const myVariable = "Hello";

• What is a callback function in JavaScript? A callback function is a function passed into another function as an argument, which is then invoked inside the outer function.

   function greet(name, callback) {
     console.log('Hello ' + name);
     callback();
   }

   greet('John', function() {
     console.log('This is a callback');
   });

• What is closure in JavaScript?
  • A closure is a function having access to the parent scope, even after the parent function has closed.

   function outer() {
     var x = 10;
     function inner() {
       console.log(x);
     }
     return inner;
   }

   var innerFunc = outer();
   innerFunc(); // Output: 10

• How do you write a for loop in JavaScript?

   for (var i = 0; i < 5; i++) {
     console.log(i);
   }

• How do you write an if-else statement in JavaScript?

var x = 10; 

if (x > 5) 
{ 
  console.log("x is greater than 5"); 
} else { 
  console.log("x is not greater than 5"); 
}

• How do you declare a function in JavaScript?

function myFunction() { 
  console.log("Hello World"); 
}

• How do you add an item to an array in JavaScript?

var arr = [1, 2, 3]; 
arr.push(4);

• How do you remove an item from an array in JavaScript?

var arr = [1, 2, 3]; 
arr.pop(); // Removes last element

• How do you check if a variable is an array in JavaScript?

var arr = [1, 2, 3]; 
console.log(Array.isArray(arr)); // true

• How can you concatenate strings in JavaScript?

var str1 = "Hello"; 
var str2 = "World"; console.log(str1 + " " + str2); // "Hello World"

• How do you declare an object in JavaScript?

var obj = { name: "John", age: 30 };

• How do you access properties of an object in JavaScript?

var obj = { name: "John", age: 30 }; 
console.log(obj.name); // "John" console.log(obj['age']); // 30

• What is the output of console.log(null == undefined) in JavaScript?
  • This will output true because == operator performs type coercion and both null and undefined are falsy values.
• What is the output of console.log(null === undefined) in JavaScript?
  • This will output false because === operator doesn’t perform type coercion.
• How do you check if a variable is undefined in JavaScript?

var myVar; if (typeof myVar === 'undefined') { console.log('myVar is undefined'); }

• What is ‘use strict’ in JavaScript?
  • 'use strict' is a directive introduced in ECMAScript 5. It helps catch common coding mistakes and “unsafe” actions by throwing exceptions.

   'use strict';
   x = 3.14; // This will throw an error because x is not declared

• What is the difference between let, const, and var?
  • var is function scoped, and if it is declared outside a function it is globally scoped. let and const are block scoped. const is a read-only reference to a value.
• What are arrow functions in JavaScript?
  • Arrow functions are a shorthand syntax for writing function expressions. They cannot be used as constructors and do not have their own this, arguments, super, or new.target.

   const sum = (a, b) => a + b;

• What are JavaScript Promises?
  • A Promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation. A Promise is in one of three states: pending, fulfilled, or rejected.

   let promise = new Promise(function(resolve, reject) {
     // the function is executed automatically when the promise is constructed

     // after 1 second signal that the job is done with the result "done"
     setTimeout(() => resolve("done"), 1000);
   });

• What is a JavaScript Promise then method?
  • The then method returns a Promise. It takes two arguments, both are callback functions for the success and failure cases respectively.

   const promise = new Promise((resolve, reject) => {
     setTimeout(() => {
       resolve("Resolved!");
     }, 1000);
   });

   promise.then(value => console.log(value)); // "Resolved!"

• What is a JavaScript Promise catch method?
  • The catch method returns a Promise and deals with rejected cases only.

   const promise = new Promise((resolve, reject) => {
     setTimeout(() => {
       reject("Rejected!");
     }, 1000);
   });

   promise.catch(reason => console.log(reason)); // "Rejected!"

• What is JavaScript `async/await`?

The async function declaration defines an asynchronous function, which returns an AsyncFunction object. The await operator is used to wait for a Promise.

   async function myFunction() {
     const promise = new Promise((resolve, reject) => {
       setTimeout(() => resolve("Hello"), 1000);
     });

     const result = await promise; // wait until the promise resolves
     console.log(result); // "Hello"
   }

   myFunction();

• What is the difference between == and === in JavaScript?
  • The == operator will compare for equality after doing any necessary type conversions. The === operator will not do the conversion, so if two values are not the same type === will simply return false.
• How do you add a property to an object in JavaScript?

   var obj = {
     name: "John",
     age: 30
   };
   obj.job = "Developer";
   console.log(obj); // {name: "John", age: 30, job: "Developer"}

• How do you delete a property from an object in JavaScript?

   var obj = {
     name: "John",
     age: 30,
     job: "Developer"
   };
   delete obj.job;
   console.log(obj); // {name: "John", age: 30}

• What is NaN in JavaScript?
  • NaN is a special value in JavaScript that stands for “Not a Number”. It is the returned value when Math functions fail or when a function trying to parse a number fails.

   console.log(0 / 0); // NaN

• How do you check if a value is NaN in JavaScript?

   console.log(isNaN(0 / 0)); // true

• What is an IIFE (Immediately Invoked Function Expression) in JavaScript?
  • An IIFE is a JavaScript function that runs as soon as it is defined.

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

• What is the difference between null and undefined in JavaScript?
  • null is an assignment value. It means no value or no object. It is not an identifier for a property of an object or for a variable itself. undefined means a variable has been declared but has not yet been assigned a value.
• How to get a random number between 1 and 100 in JavaScript?

   var randomNum = Math.floor(Math.random() * 100) + 1;
   console.log(randomNum);

• How can you convert a string to an integer in JavaScript?

   var str = "123";
   var int = parseInt(str);
   console.log(int); // 123

• How can you convert a string to a number in JavaScript?

   var str = "123.45";
   var num = parseFloat(str);
   console.log(num); // 123.45

• What are JavaScript data types?
  • JavaScript data types include Number, String, Boolean, Object, Function, Null, Undefined, and Symbol (in ES6).
• How can you get the type of a variable in JavaScript?

   var str = "Hello";
   console.log(typeof str); // "string"

• What is JSON in JavaScript?
  • JSON (JavaScript Object Notation) is a lightweight data-interchange format that is easy for humans to read and write and easy for machines to parse and generate. It is a text format that is completely language independent but uses conventions that are familiar to programmers of the C-family of languages, including C, C++, C#, Java, JavaScript, Perl, Python, and many others.
• How do you parse JSON in JavaScript?

   var json = '{"name":"John", "age":30, "city":"New York"}';
   var obj = JSON.parse(json);
   console.log(obj.name); // "John"

• How do you convert an object to a JSON string in JavaScript?

   var obj = {name: "John", age: 30, city: "New York"};
   var json = JSON.stringify(obj);
   console.log(json); // '{"name":"John", "age":30, "city":"New York"}'

• How do you create a date object in JavaScript?

   var date = new Date();
   console.log(date);

• How do you get the current time in JavaScript?

   var date = new Date();
   console.log(date.getTime());

• How do you get the current date in JavaScript?

   var date = new Date();
   console.log(date.toDateString());

• What are JavaScript events?
  • JavaScript events are actions that can be detected by JavaScript. For example, a user clicking a button is an event.
• How do you prevent the default action of an event in JavaScript?

   element.addEventListener('click', function(event) {
     event.preventDefault();
     // do something
   });

• How do you attach an event handler to an element in JavaScript?

   var button = document.querySelector('button');
   button.addEventListener('click', function() {
     console.log('Button clicked');
   });

• How do you get the value of an input field in JavaScript?

   var input = document.querySelector('input');
   console.log(input.value);

• How do you change the content of an HTML element in JavaScript?

   var element = document.querySelector('p');
   element.textContent = 'New content';

I hope these questions will help you in learning JavaScript. Happy coding!