Here are 50 simple Javascript questions along with their answers and code examples:<\/p>\n\n\n\n
console.log(5 + \"5\")<\/code> in JavaScript?<\/strong> \n\n- This will output
55<\/code>. Because one operand is a string, JavaScript will convert the other operand to a string before performing the concatenation.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- What is the output of
console.log(5 - \"5\")<\/code> in JavaScript?<\/strong> \n\n- This will output
0<\/code>. JavaScript performs type coercion and converts the string \"5\"<\/code> to the number 5<\/code>, then does the subtraction.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- What is the output of
console.log(5 == \"5\")<\/code> in JavaScript?<\/strong> \n\n- This will output
true<\/code>. The double equals ==<\/code> operator performs type coercion if the types on both sides are different.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- What is the output of
console.log(5 === \"5\")<\/code> in JavaScript?<\/strong> \n\n- This will output
false<\/code>. The triple equals ===<\/code> operator does not perform type coercion.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- Write a function to reverse a string in JavaScript.<\/strong><\/li>\n<\/ul>\n\n\n\n
function reverseString(str) {\n return str.split('').reverse().join('');\n }<\/pre><\/div>\n\n\n\n\n- How do you declare a variable in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var myVariable = "Hello";\n let myVariable = "Hello";\n const myVariable = "Hello";<\/pre><\/div>\n\n\n\n
\n- What is a callback function in JavaScript?<\/strong> A callback function is a function passed into another function as an argument, which is then invoked inside the outer function.<\/li>\n<\/ul>\n\n\n\n
function greet(name, callback) {\n console.log('Hello ' + name);\n callback();\n }\n\n greet('John', function() {\n console.log('This is a callback');\n });<\/pre><\/div>\n\n\n\n\n- What is closure in JavaScript?<\/strong> \n
\n- A closure is a function having access to the parent scope, even after the parent function has closed.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n
function outer() {\n var x = 10;\n function inner() {\n console.log(x);\n }\n return inner;\n }\n\n var innerFunc = outer();\n innerFunc(); \/\/ Output: 10<\/pre><\/div>\n\n\n\n\n- How do you write a for loop in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
for (var i = 0; i < 5; i++) {\n console.log(i);\n }<\/pre><\/div>\n\n\n\n\n- How do you write an if-else statement in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var x = 10; \n\nif (x > 5) \n{ \n console.log("x is greater than 5"); \n} else { \n console.log("x is not greater than 5"); \n}<\/pre><\/div>\n\n\n\n\n- How do you declare a function in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
function myFunction() { \n console.log("Hello World"); \n}<\/pre><\/div>\n\n\n\n\n- How do you add an item to an array in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var arr = [1, 2, 3]; \narr.push(4);<\/pre><\/div>\n\n\n\n
\n- How do you remove an item from an array in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var arr = [1, 2, 3]; \narr.pop(); \/\/ Removes last element<\/pre><\/div>\n\n\n\n
\n- How do you check if a variable is an array in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var arr = [1, 2, 3]; \nconsole.log(Array.isArray(arr)); \/\/ true<\/pre><\/div>\n\n\n\n
\n- How can you concatenate strings in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var str1 = "Hello"; \nvar str2 = "World"; console.log(str1 + " " + str2); \/\/ "Hello World"<\/pre><\/div>\n\n\n\n
\n- How do you declare an object in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var obj = { name: "John", age: 30 };<\/pre><\/div>\n\n\n\n\n- How do you access properties of an object in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var obj = { name: "John", age: 30 }; \nconsole.log(obj.name); \/\/ "John" console.log(obj['age']); \/\/ 30<\/pre><\/div>\n\n\n\n\n- What is the output of
console.log(null == undefined)<\/code> in JavaScript?<\/strong> \n\n- This will output
true<\/code> because ==<\/code> operator performs type coercion and both null<\/code> and undefined<\/code> are falsy values.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- What is the output of
console.log(null === undefined)<\/code> in JavaScript?<\/strong> \n\n- This will output
false<\/code> because ===<\/code> operator doesn’t perform type coercion.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- How do you check if a variable is undefined in JavaScript?<\/strong> <\/li>\n<\/ul>\n\n\n\n
var myVar; if (typeof myVar === 'undefined') { console.log('myVar is undefined'); }<\/pre><\/div>\n\n\n\n\n- What is ‘use strict’ in JavaScript?<\/strong> \n
\n'use strict'<\/code> is a directive introduced in ECMAScript 5. It helps catch common coding mistakes and “unsafe” actions by throwing exceptions.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n 'use strict';\n x = 3.14; \/\/ This will throw an error because x is not declared<\/pre><\/div>\n\n\n\n
\n- What is the difference between
let<\/code>, const<\/code>, and var<\/code>?<\/strong> \n\nvar<\/code> is function scoped, and if it is declared outside a function it is globally scoped. let<\/code> and const<\/code> are block scoped. const<\/code> is a read-only reference to a value.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- What are arrow functions in JavaScript?<\/strong> \n
\n- Arrow functions are a shorthand syntax for writing function expressions. They cannot be used as constructors and do not have their own
this<\/code>, arguments<\/code>, super<\/code>, or new.target<\/code>.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n const sum = (a, b) => a + b;<\/pre><\/div>\n\n\n\n
\n- What are JavaScript Promises?<\/strong> \n
\n- 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.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n
let promise = new Promise(function(resolve, reject) {\n \/\/ the function is executed automatically when the promise is constructed\n\n \/\/ after 1 second signal that the job is done with the result "done"\n setTimeout(() => resolve("done"), 1000);\n });<\/pre><\/div>\n\n\n\n\n- What is a JavaScript Promise
then<\/code> method?<\/strong> \n\n- The
then<\/code> method returns a Promise. It takes two arguments, both are callback functions for the success and failure cases respectively.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n const promise = new Promise((resolve, reject) => {\n setTimeout(() => {\n resolve("Resolved!");\n }, 1000);\n });\n\n promise.then(value => console.log(value)); \/\/ "Resolved!"<\/pre><\/div>\n\n\n\n\n- What is a JavaScript Promise
catch<\/code> method?<\/strong> \n\n- The
catch<\/code> method returns a Promise and deals with rejected cases only.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n const promise = new Promise((resolve, reject) => {\n setTimeout(() => {\n reject("Rejected!");\n }, 1000);\n });\n\n promise.catch(reason => console.log(reason)); \/\/ "Rejected!"<\/pre><\/div>\n\n\n\n\n- What is JavaScript `async\/await`?<\/strong><\/li>\n<\/ul>\n\n\n\n
The async<\/code> function declaration defines an asynchronous function, which returns an AsyncFunction object. The await<\/code> operator is used to wait for a Promise.<\/p>\n\n\n\n async function myFunction() {\n const promise = new Promise((resolve, reject) => {\n setTimeout(() => resolve("Hello"), 1000);\n });\n\n const result = await promise; \/\/ wait until the promise resolves\n console.log(result); \/\/ "Hello"\n }\n\n myFunction();<\/pre><\/div>\n\n\n\n\n- What is the difference between
==<\/code> and ===<\/code> in JavaScript?<\/strong> \n\n- The
==<\/code> operator will compare for equality after doing any necessary type conversions. The ===<\/code> operator will not do the conversion, so if two values are not the same type ===<\/code> will simply return false<\/code>.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- How do you add a property to an object in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var obj = {\n name: "John",\n age: 30\n };\n obj.job = "Developer";\n console.log(obj); \/\/ {name: "John", age: 30, job: "Developer"}<\/pre><\/div>\n\n\n\n\n- How do you delete a property from an object in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var obj = {\n name: "John",\n age: 30,\n job: "Developer"\n };\n delete obj.job;\n console.log(obj); \/\/ {name: "John", age: 30}<\/pre><\/div>\n\n\n\n\n- What is
NaN<\/code> in JavaScript?<\/strong> \n\nNaN<\/code> 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.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n console.log(0 \/ 0); \/\/ NaN<\/pre><\/div>\n\n\n\n
\n- How do you check if a value is
NaN<\/code> in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n console.log(isNaN(0 \/ 0)); \/\/ true<\/pre><\/div>\n\n\n\n
\n- What is an IIFE (Immediately Invoked Function Expression) in JavaScript?<\/strong> \n
\n- An IIFE is a JavaScript function that runs as soon as it is defined.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n
(function() {\n console.log('Hello World');\n })();<\/pre><\/div>\n\n\n\n\n- What is the difference between null and undefined in JavaScript?<\/strong> \n
\nnull<\/code> 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<\/code> means a variable has been declared but has not yet been assigned a value.<\/li>\n<\/ul>\n<\/li>\n\n\n\n- How to get a random number between 1 and 100 in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var randomNum = Math.floor(Math.random() * 100) + 1;\n console.log(randomNum);<\/pre><\/div>\n\n\n\n
\n- How can you convert a string to an integer in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var str = "123";\n var int = parseInt(str);\n console.log(int); \/\/ 123<\/pre><\/div>\n\n\n\n
\n- How can you convert a string to a number in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var str = "123.45";\n var num = parseFloat(str);\n console.log(num); \/\/ 123.45<\/pre><\/div>\n\n\n\n
\n- What are JavaScript data types?<\/strong> \n
\n- JavaScript data types include Number, String, Boolean, Object, Function, Null, Undefined, and Symbol (in ES6).<\/li>\n<\/ul>\n<\/li>\n\n\n\n
- How can you get the type of a variable in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var str = "Hello";\n console.log(typeof str); \/\/ "string"<\/pre><\/div>\n\n\n\n
\n- What is JSON in JavaScript?<\/strong> \n
\n- 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.<\/li>\n<\/ul>\n<\/li>\n\n\n\n
- How do you parse JSON in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var json = '{"name":"John", "age":30, "city":"New York"}';\n var obj = JSON.parse(json);\n console.log(obj.name); \/\/ "John"<\/pre><\/div>\n\n\n\n\n- How do you convert an object to a JSON string in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var obj = {name: "John", age: 30, city: "New York"};\n var json = JSON.stringify(obj);\n console.log(json); \/\/ '{"name":"John", "age":30, "city":"New York"}'<\/pre><\/div>\n\n\n\n\n- How do you create a date object in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var date = new Date();\n console.log(date);<\/pre><\/div>\n\n\n\n
\n- How do you get the current time in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var date = new Date();\n console.log(date.getTime());<\/pre><\/div>\n\n\n\n
\n- How do you get the current date in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
var date = new Date();\n console.log(date.toDateString());<\/pre><\/div>\n\n\n\n
\n- What are JavaScript events?<\/strong> \n
\n- JavaScript events are actions that can be detected by JavaScript. For example, a user clicking a button is an event.<\/li>\n<\/ul>\n<\/li>\n\n\n\n
- How do you prevent the default action of an event in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n
element.addEventListener('click', function(event) {\n event.preventDefault();\n \/\/ do something\n });<\/pre><\/div>\n\n\n\n\n- How do you attach an event handler to an element in JavaScript?<\/strong><\/li>\n<\/ul>\n\n\n\n