UI Developer Interview Questions with answers updated in 2022 for User Interface Developers , Web Designers , Frontend Developers and Web Developers with HTML5, CSS3, JavaScript, JS ES6, ES7, ES8 and JQuery Interview Questions. These UI interview questions are based on recent interviews conducted by our candidates in Delhi NCR, or sourced from Linkedin, Stack overflow, Github and Glassdoor.


UI Developer

UI Developer profile comes under frontend developers who build User Interface or Front End of a website. Frontend is very vast now. Frontend also includes framework like Angular, backbone, knockout or libraries like Vue JS, React JS with Redux etc. To build User Interface of a website, a UI Developer must know HTML, CSS and JavaScript or jQuery used to build User Interface of a website. UI Developer / Frontend Developer is one of the fastest growing job profile in IT Sector.

UI Developers are among the top highest paid in IT Sector. An experienced UI Developer can earn upto $1,25,000 PM.

Source - Forbes Survey, 2016

UI Interview Questions

UI developer interview questions with answers. If you are looking for latest UI Interview questions with answers, you are at right place. Here are some commonly asked UI Developer Interview Questions with answers.

what is difference between UI and UX

UI means USER Interface and UX means User Experience. UI is mainly related to look and feel, colors combinations, fonts selections and layout. But UX is related to how UI is used, i.e. usability, wire-framing, interaction with UI. UI focus on coding skills like HTML, CSS and Javascript, but UX focus on interaction with UI.

Design of button is UI, but interaction with the button is UX.

How to test website performance.

Website performance play a major role. As per Google Lighthouse ( included in chrome browser ) and Think With Google website, the performance of a website is excellent if its loading time is under 4 seconds. A website is fair if loading time is 4 to 5 seconds. But a website is poor if loading time is more than 8 seconds.

Even Chrome Lighthouse test available in Google Chrome, Microsoft Edge and Chromium browser is good for website performance and Usability testing.

website speed test and performance
website speed test

Use Chrome Lighthouse, a build in tool in chrome, edge chromium and chromium browser to test website speed, performance, usability and SEO of website.

HTML Interview Questions

What is the difference between XHTML, HTML4 and HTML5?

XHTML was the cleaner and stricter version of HTML4. Whereas HTML5 is the latest Web Standard with CSS & JavaScript. HTML5 is the latest version of HTML with semantic elements, audio video, new form elements and attributes, Web APIs and CSS3.

  HTML4 XHTML HTML5
Year 1998 2000 2009
Includes html only html only html, css and javascript
Tags both upper and lower case lowercase both upper and lower case ( recommended )
Boolean Attributes supported not supported supported
presentational tags & attributes supported supported not supported, use css
marquee supported supported not supported

Does HTML needs compiler?

HTML, CSS and JS are browser interpreted technologies. Whereas Asp.net, Java and PHP are not browser interpreted. They run on server and needs a compiler or interpreter which can convert their code into machine code. That's why HTML, CSS and JavaScript are also called frontend technologies. Browsers parse html, css and Compile JavaScript on runtime using JIT Compiler.

What is Semantic HTML?

HTML5 introduced semantic tags like header, nav, article, section, aside, footer and figcaption. These tags convey the containing content. Whereas in html4, div tag was used to create partitions using descriptive classes. Semantic Elements results better in search results and social media crawling.

Which is best HTML debugging tool.

We use chrome, Firefox and Firebug (removed from Firefox but available in Firefox Developer edition) Dev tools as they are browser inbuilt. W3C Validation is best HTML/CSS Debugging tools to validate. Chrome Lighthouse is also recommended for UI Testing and SEO.

How to use HTML5 New tags in IE8 and lesser.

HTML5 semantic tags like, header, nav, article, section, aside, footer and figcaption are not supported in HTML4 based browsers like IE8 and lesser. We can use html5shiv.js to run these elements in older browsers.
→ See More html5 tags in IE8

What is web accessibility?

Web accessibility means a web for differently abled or disabled people. HTML5 introduced, role attribute and WAI-ARIA so that even disable people can interact, understand and navigate a website properly.

Difference between <b> and <strong> tag?

<b> is a presentational element used to give bolder look to text. Whereas <strong> gives bolder look and strong importance in search results.

Difference between <i> and <em> tag?

<i> is a presentational element used to give italic look to text. Whereas <em> gives italic look and emphasis in search results.

Difference between <article> and <section> tag?

Article and section are both Semantic elements in HTML5.
Article is used for independent content with heading, like Blog post, Review, etc.
Section is used for generic section of a page, like sections in article. Both Article and section must include heading or subheading.

What is Meta Viewport?

Apple introduced Meta Viewport first in safari to adjust viewport size in IOS. Soon others mobile browsers starts supporting same rule. Although its non-standard, but still works on all browsers. For responsive websites, meta viewport is compulsory.

CSS Interview Questions

Difference between reset.css and normalize css?

reset.css removes all build-in browsers styling like, margin , padding and border . But normalize.css gives a common book like appearance, like bold heading, margin between paragraphs, Screen friendly fonts, etc.

What is CSS Box Model?

CSS Box Model includes content, margin, padding, and border. Total border-box width is equal to width of content + padding + border. These properties occupies space on css box model.
→ See More CSS Box Model

How to check coding errors in css?

Open your webpage in Firefox. Press Ctrl + shift + j. You will see all security, javascript and css related issues. You can also use W3C validator for HTML and CSS related issues. Even W3C validation can check css errors now.

Difference between display none and visibility hidden ?

Visibility:hidden hide the content from user but retain space. Whereas Display:none hide the content and remove space.

How to center align a div tag in css?

Div is block level element. By default, div occupy 100% width of parent element. To center align a div in css , use


<style>
 .wrap{ 
     width:960px;
     margin:auto;
 }
 </style>
 <div class="wrap"<
     // content for wrap div
 </div>

How to vertically align a div in css?

To align div in vertical center , use display:table-cell with vertical-align:middle. Make sure the height of div is more than content and display of parent div is table.


<style>
 .box{ 
     width:400px;
     height:200px;
     display:table;
     vertical-align:middle;
 }
 .box p{
    display:table-cell; 
 }
 </style>
 <div class="box"<
    <p>vertical center text</p>
 </div>

Difference between Position Relative, Absolute and Fixed

Position relative can move an element with respect to its current position. Position absolute can move an element with respect to its current position or parent elements position. Absolute Elements doesn't occupy space in DOM Flow. Position fixed gives fixed position to an element with respect to device screen. These elements doesn't move on scrolling. Fixed Elements also doesn't occupy space in DOM Flow.
See More CSS Positions

What are vendor specific prefixes like webkit?

vendor specific prefixes are used mainly with CSS3 properties to run them in all browsers. Chrome, Edge, Safari and Firefox use webkit engine. But earlier firefox use -moz prefix, opera use -o prefix and Internet Explorer support -ms prefix. For latest browsers, only webkit is required now.


JavaScript Interview Questions

JavaScript Interview Questions for web developer, UI Developers, Frontend Developers and Web Designers with ES6, ES7, ES8 and Advance modules like OOPS, AJAX, Web APIs.

What is JavaScript

JavaScript is prototype (object based) based general purpose programming language used in web application and developer. JS is also known as the programming language of web. JavaScript was primarily build for web browser only. But now, JavaScript is also used outside a web browser using Node JS.

Is JavaScript Case Sensitive?

Yes, JavaScript is case-sensitive. But HTML and CSS are in case sensitive.

Is JavaScript Compiles or interpreted?

Earlier JS engines interpret JS code to machine, but in 2009, Firefox 3.6 was first browser to introduce compilation using JIT compiler or runtime compilation to improve JS Execution. Now all browsers use JIT Compilers ( except Internet Explorer ).

What is "use strict" in javascript?

Javascript introduced strict mode in ES5. Strict Mode is recommended for detecting logical errors earlier. Here are the things which are not allowed in strict mode.

<script>
    "use strict";
    let x=010;                  // octal notation
     y=010;                     // no variable declaration

</script>

What is difference between == and === operator?

== is comparison operator , whereas === is Strict equality. == checks value only, === check value and datatype. For exp

        5=="5"     // true, 
        5==="5"    // false.
    
        1==true     // true, 
        1===true    // false.
    
        undefined==null     // true, 
        undefined===null     // false.
    

What is output of "20" + 20?

"2020". First 20 is string type, second is numeric, for addition, both should be numeric type.

What is output of "20" + 20 + 20 and "20" + ( 20 + 20)?

"20" + 20 + 20 = "202020" but "20" + ( 20 + 20) is "2040".

How to convert "20" to number?

Three ways to convert string into number


    var x="20";
    x=+x;           //  20
    Number(x)       //  20 
    parseInt(x)     //  20
    parseFloat(x)   //  20
        

What is an AJAX request and give a simple example of where an AJAX request would be used.

AJAX is Asynchronous JavaScript and XML . It is client side process written in Javascript used to get or post data from remote server without reloading page JavaScript AJAX .

What is Variable Hoisting?

A variable is declared first and then called. If variable is declared on bottom and called earlier, value will be undefined. Only assigned values are hoisted in Javascript, like string, numbers etc. Undefined and function declaration can be called before declaration as they are not hoisted, and there is no assignment operator (=) in both. JavaScript Variables

Callback Function

A function that is passed as argument to another function is called callback function in javascript. Callback functions are among High Order Functions. JavaScript Callback Functions.

What is closure in javascript

A var variable declared inside a function, i.e. local variable can also be accessed inside child function. This technique in JavaScript is called closures. See example.


function outer(){
    var i=2;            // for both outer and inner functions
    
    function inner(){
        var j=3;        // for inner function only
        var k=i+j;
        console.log(k);        // 5
    }
}

Event Delegation, Bubbling and Capturing Events.

Event Propagation are the order that event fire on element. Bubbling is when an event fire on the element and then bubble up DOM Tree. Means First child and then parent will call. Capturing is exactly opposite. First parent node is called and then propagates down towards target element. Event Bubbling Vs capturing

What is Block Scope?

let and const are block scoped in javascript. A curly brackets is a block used in if, for, while etc. let and const can be used only inside block, unlike var (global scope). See more, JavaScript variables

What are Arrow Functions?

Arrow functions are function declaration in es6 but instead of using function keyword, we use ()={}. They are mainly used as callbacks to write better code. Also this keyword is not current object in arrow functions.


    let a=(x,y)=>{return x*y} ;
    let b=x=>2*x;
    [1,3,9,12].sort((a,b)=>a-b);

Jquery Interview Questions

Use of Jquery ready function.

Jquery recommends use of $(document).ready(function(){ }) or $(function(){ }) in script tag to write jquery syntax. Jquery ready function execute Jquery functions after all html elements are loaded, before images, iframe, ads are loaded.


$(document).ready(function(){
    // jquery code
});
            

Jquery Ready Functions Short Form


$(function(){
    // jquery code
});
            

Difference between Jquery ready function and window onload.

window.onload is a javascript function whereas $(document).ready is a Jquery function. Jquery ready function loads content slightly faster than javascript window.onload function. window.onload wait for all DOM contents including media to load first and then executed, whereas Jquery ready function wait for only html elements, not media, thus load fast.

Various Jquery versions.

Jquery 1 was the first stable release of Jquery. Many websites still use Jquery 1 as Jquery 1 supports all major browsers, including IE 8, 7 and 6.

Jquery Versions and difference
Version Release Size Browser Support Description
1.0 Aug 2006 IE 6 and above First Stable Version
1.12 Jan 2018 95 kb IE 6 and above Latest Stable Version
2 Apr 2013 81 kb IE 9 and above 10% lighter than Jquery 1, no support in IE 8 and below.
3 June 2016 86 kb IE 8 and above Latest Stable Version with HTML5 Support and Securities.
For IE 8 and below, use Jquery 1 , the latest stable version of Jquery 1 is Jquery 1.12.4 .