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#efefef}.footer{padding-top:3rem}.footer-text{font-size:.8rem}</style></head><body class=""><div class="topbar"></div><div class="container"><a class="skip-link" href="#main-content">Skip to main content</a><nav class="nav"><ul class="navigation" id="navigation"><li class="navigation-item-emoji navigation-item-home"><a class="navigation-anchor" href="/'Front-end-Developer-Interview-Questions/'/" title="Home"><em>★</em><span>Front-end Developer Interview Questions</span></a></li><li class="navigation-item-emoji navigation-item-translations"><a class="navigation-anchor" href="/'Front-end-Developer-Interview-Questions/'/translations" title="Translations"><em>🌐</em><span>Translations</span></a></li><li class="navigation-item navigation-questions-label"><button class="navigation-toggle" id="navigation-toggle">Questions:</button></li><li class="navigation-item navigation-questions"><ul class="navigation navigation-questions-list"><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/general-questions/" title="General Questions">General</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/html-questions/" title="HTML Questions">HTML</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/css-questions/" title="CSS Questions">CSS</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor current" href="/'Front-end-Developer-Interview-Questions/'/questions/javascript-questions/" title="JavaScript Questions">JavaScript</a></li><li class="navigation-item"><a rel="external noreferrer noopener" target="_blank" class="navigation-anchor navigation-questions-anchor" href="https://scottaohara.github.io/accessibility_interview_questions/" title="Accessibility Questions (external link)">A11y</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/testing-questions/" title="Testing Questions">Testing</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/performance-questions/" title="Performance Questions">Performance</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/network-questions/" title="Network Questions">Network</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/coding-questions/" title="Coding Questions">Coding</a></li><li class="navigation-item"><a class="navigation-anchor navigation-questions-anchor" href="/'Front-end-Developer-Interview-Questions/'/questions/fun-questions/" title="Fun Questions">Fun</a></li></ul></li></ul></nav><main id="main-content" tabindex="-1"><header class="header"><h1 class="header-big-title">JavaScript Questions</h1></header><div class="content"><ul><li>Explain event delegation.</li><li>Explain how <code>this</code> works in JavaScript.<ul><li>Can you give an example of one of the ways that working with <code>this</code> has changed in ES6?</li></ul></li><li>Explain how prototypal inheritance works.</li><li>What’s the difference between a variable that is: <code>null</code>, <code>undefined</code> or undeclared?<ul><li>How would you go about checking for any of these states?</li></ul></li><li>What is a closure, and how/why would you use one?</li><li>What language constructions do you use for iterating over object properties and array items?</li><li>Can you describe the main difference between the <code>Array.forEach()</code> loop and <code>Array.map()</code> methods and why you would pick one versus the other?</li><li>What’s a typical use case for anonymous functions?</li><li>What’s the difference between host objects and native objects?</li><li>Explain the difference between: <code>function Person(){}</code>, <code>var person = Person()</code>, and <code>var person = new Person()</code>?</li><li>Explain the differences on the usage of <code>foo</code> between <code>function foo() {}</code> and <code>var foo = function() {}</code></li><li>Can you explain what <code>Function.call</code> and <code>Function.apply</code> do? What’s the notable difference between the two?</li><li>Explain <code>Function.prototype.bind</code>.</li><li>What’s the difference between feature detection, feature inference, and using the UA string?</li><li>Explain “hoisting”.</li><li>Describe event bubbling.</li><li>Describe event capturing.</li><li>What’s the difference between an “attribute” and a “property”?</li><li>What are the pros and cons of extending built-in JavaScript objects?</li><li>What is the difference between <code>==</code> and <code>===</code>?</li><li>Explain the same-origin policy with regards to JavaScript.</li><li>Why is it called a Ternary operator, what does the word “Ternary” indicate?</li><li>What is strict mode? What are some of the advantages/disadvantages of using it?</li><li>What are some of the advantages/disadvantages of writing JavaScript code in a language that compiles to JavaScript?</li><li>What tools and techniques do you use debugging JavaScript code?</li><li>Explain the difference between mutable and immutable objects.<ul><li>What is an example of an immutable object in JavaScript?</li><li>What are the pros and cons of immutability?</li><li>How can you achieve immutability in your own code?</li></ul></li><li>Explain the difference between synchronous and asynchronous functions.</li><li>What is event loop?<ul><li>What is the difference between call stack and task queue?</li></ul></li><li>What are the differences between variables created using <code>let</code>, <code>var</code> or <code>const</code>?</li><li>What are the differences between ES6 class and ES5 function constructors?</li><li>Can you offer a use case for the new arrow <code>=></code> function syntax? How does this new syntax differ from other functions?</li><li>What advantage is there for using the arrow syntax for a method in a constructor?</li><li>What is the definition of a higher-order function?</li><li>Can you give an example for destructuring an object or an array?</li><li>Can you give an example of generating a string with ES6 Template Literals?</li><li>Can you give an example of a curry function and why this syntax offers an advantage?</li><li>What are the benefits of using <code>spread syntax</code> and how is it different from <code>rest syntax</code>?</li><li>How can you share code between files?</li><li>Why you might want to create static class members?</li><li>What is the difference between <code>while</code> and <code>do-while</code> loops in JavaScript?</li><li>What is a promise? Where and how would you use promise?</li></ul><h2 id="coding-questions" tabindex="-1"><a class="header-anchor" href="#coding-questions"></a> Coding questions</h2><ul><li>Make this work:</li></ul><pre class="language-javascript"><code class="language-javascript"><span class="token function">duplicate</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// [1,2,3,4,5,1,2,3,4,5]</span></code></pre><ul><li>Create a for loop that iterates up to <code>100</code> while outputting <strong>“fizz”</strong> at multiples of <code>3</code>, <strong>“buzz”</strong> at multiples of <code>5</code> and <strong>“fizzbuzz”</strong> at multiples of <code>3</code> and <code>5</code></li><li>What will be returned by each of these?</li></ul><pre class="language-javascript"><code class="language-javascript">console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"hello"</span> <span class="token operator">||</span> <span class="token string">"world"</span><span class="token punctuation">)</span><br>console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"foo"</span> <span class="token operator">&&</span> <span class="token string">"bar"</span><span class="token punctuation">)</span></code></pre><ul><li>Write an immediately invoked function expression (IIFE)</li></ul></div><div class="content-edit"><a rel="external noreferrer noopener" target="_blank" href="https://github.com/h5bp/Front-end-Developer-Interview-Questions/tree/master/src/questions/javascript-questions.md">Edit this page</a></div></main><footer class="footer"><p class="footer-text">Copyright © 2012 - 2021. 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