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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" 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 current" 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">Coding Questions</h1></header><div class="content"><p>Question: What is the value of <code>foo</code>?</p><pre class="language-javascript"><code class="language-javascript"><span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token number">10</span> <span class="token operator">+</span> <span class="token string">'20'</span><span class="token punctuation">;</span></code></pre><p>Question: What will be the output of the code below?</p><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 number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span> <span class="token operator">==</span> <span class="token number">0.3</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: How would you make this work?</p><pre class="language-javascript"><code class="language-javascript"><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">2</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 comment">// 7</span>
<span class="token function">add</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</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 comment">// 7</span></code></pre><p>Question: What value is returned from the following statement?</p><pre class="language-javascript"><code class="language-javascript"><span class="token string">"i'm a lasagna hog"</span><span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What is the value of <code>window.foo</code>?</p><pre class="language-javascript"><code class="language-javascript"><span class="token punctuation">(</span> window<span class="token punctuation">.</span>foo <span class="token operator">||</span> <span class="token punctuation">(</span> window<span class="token punctuation">.</span>foo <span class="token operator">=</span> <span class="token string">"bar"</span> <span class="token punctuation">)</span> <span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What is the outcome of the two alerts below?</p><pre class="language-javascript"><code class="language-javascript"><span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token string">"Hello"</span><span class="token punctuation">;</span>
<span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">var</span> bar <span class="token operator">=</span> <span class="token string">" World"</span><span class="token punctuation">;</span>
<span class="token function">alert</span><span class="token punctuation">(</span>foo <span class="token operator">+</span> bar<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">alert</span><span class="token punctuation">(</span>foo <span class="token operator">+</span> bar<span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What is the value of <code>foo.length</code>?</p><pre class="language-javascript"><code class="language-javascript"><span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
foo<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
foo<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What is the value of <code>foo.x</code>?</p><pre class="language-javascript"><code class="language-javascript"><span class="token keyword">var</span> foo <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">n</span><span class="token operator">:</span> <span class="token number">1</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> bar <span class="token operator">=</span> foo<span class="token punctuation">;</span>
foo<span class="token punctuation">.</span>x <span class="token operator">=</span> foo <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token literal-property property">n</span><span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">;</span></code></pre><p>Question: What does the following code print?</p><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">'one'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'two'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Promise<span class="token punctuation">.</span><span class="token function">resolve</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'three'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'four'</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What is the difference between these four promises?</p><pre class="language-javascript"><code class="language-javascript"><span class="token function">doSomething</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token function">doSomethingElse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">doSomething</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">doSomethingElse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">doSomething</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token function">doSomethingElse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">doSomething</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span>doSomethingElse<span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: What will the code below output to the console and why?</p><pre class="language-javascript"><code class="language-javascript"><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">var</span> a <span class="token operator">=</span> b <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"a defined? "</span> <span class="token operator">+</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> a <span class="token operator">!==</span> <span class="token string">'undefined'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">"b defined? "</span> <span class="token operator">+</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> b <span class="token operator">!==</span> <span class="token string">'undefined'</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre><p>Question: Consider the two functions below. Will they both return the same thing? Why or why not?</p><pre class="language-javascript"><code class="language-javascript"><span class="token keyword">function</span> <span class="token function">foo1</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token punctuation">{</span>
<span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">"hello"</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">foo2</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span>
<span class="token punctuation">{</span>
<span class="token literal-property property">bar</span><span class="token operator">:</span> <span class="token string">"hello"</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span></code></pre></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/coding-questions.md">Edit this page</a></div></main><footer class="footer"><p class="footer-text">Copyright &copy; 2012 - 2024. Contributors to <a href="https://github.com/h5bp/Front-end-Developer-Interview-Questions/blob/main/CONTRIBUTORS.md">Front-end-Developer-Interview-Questions.</a><br>Curious about the project? <a href="/Front-end-Developer-Interview-Questions/about/">Read more about here</a>.</p></footer></div><script>console.log("%cA black belt is a white belt that didn't quit.","color: #fff; font-size: 20px; text-shadow: 1px 1px 1px #000; font-family: serif;");var navToggle=document.getElementById("navigation-toggle"),nav=document.getElementById("navigation");navToggle.addEventListener("click",function(){nav.classList.toggle("open")}),window.addEventListener("resize",function(){document.width>960?nav.classList.remove("active"):(nav.classList.add("no-motion"),setTimeout(function(){nav.classList.remove("no-motion")},250))});</script></body></html>
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