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<h1 style="text-align: center;">javascript interview questions</h1>
<h2>[WHAT]</h2>
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<li>]&nbsp;23 questions by toptal -</li>
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<h2>[WHY]</h2>
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<li>]&nbsp;learning javascript&nbsp;- intermediate level</li>
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<h2>[WHERE]</h2>
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<li>] READ THE FULL ARTICLE</li>
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<li>] <a href="http://www.toptal.com/javascript/interview-questions" target="_blank">http://www.toptal.com/javascript/interview-questions</a></li>
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<h2>[WHEN]</h2>
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<li>]</li>
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<h2>[EXAMPLE]</h2>
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<li>]</li>
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<h2>[HOW-TO]</h2>
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<li>]</li>
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<h2>[REFERENCE]</h2>
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<li>]</li>
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1] what result will the following code produce
&nbsp;(function(){ &nbsp; var a = b = 3; })(); console.log("a defined? " + (typeof a !== 'undefined')); console.log("b defined? " + (typeof b !== 'undefined'));
ME
typeof a(number) !== 'undefined' // true, typeof b(number) !== 'undefined) // true
ANS:
Since both <code>a</code> and <code>b</code> are defined within the enclosing scope of the function, and since the line they are on begins with the <code>var</code> keyword, most JavaScript developers would expect <code>typeof a</code> and <code>typeof b</code> to both be undefined in the above example.
However, that is not the case. The issue here is that most developers incorrectly understand the statement <code>var a = b = 3;</code> to be shorthand for:
<pre><code class=" hljs cs">var b = 3; var a = b; </code></pre>
But in fact, <code>var a = b = 3;</code> is actually shorthand for:
<pre><code class=" hljs fix">b = 3; var a = b; </code></pre>
As a result (if you are not using strict mode), the output of the code snippet would be:
<pre><code class=" hljs ruby">a defined? false b defined? true </code></pre>
But how can <code>b</code> be defined outside of the scope of the enclosing function? Well, since the statement <code>var a = b = 3;</code> is shorthand for the statements <code>b = 3;</code> and <code>var a = b;</code>, <code>b</code> ends up being a global variable (since it is not preceded by the <code>var</code> keyword) and is therefore still in scope even outside of the enclosing function.
Note that, in strict mode (i.e., with <a href="http://www.w3schools.com/js/js_strict.asp" target="_blank"><code>use strict</code></a>), the statement <code>var a = b = 3;</code> will generate a runtime error of <code>ReferenceError: b is not defined</code>, thereby avoiding any headfakes/bugs that might othewise result. (Yet another prime example of why you should use <code>use strict</code> as a matter of course in your cod
2] What is a potential pitfall with using <code>typeof bar === "object"</code> to determine if <code>bar</code> is an object? How can this pitfall be avoided?
null is also considered an object, therefore an object that has been declared but not assigned any value would be considered an object
&nbsp;ME: no
&nbsp;ANS:
Although <code>typeof bar === "object"</code> is a reliable way of checking if <code>bar</code> is an object, the surprising gotcha in JavaScript is that <code>null</code> is also considered an object!
Therefore, the following code will, to the surprise of most developers, log <code>true</code> (not <code>false</code>) to the console:
<pre><code class=" hljs coffeescript">var bar = null; console.log(typeof bar === "object"); // logs true! </code></pre>
As long as one is aware of this, the problem can easily be avoided by also checking if <code>bar</code> is <code>null</code>:
<pre><code class=" hljs coffeescript">console.log((bar !== null) &amp;&amp; (typeof bar === "object")); // logs false </code></pre>
To be entirely thorough in our answer, there are two other things worth noting:
First, the above solution will return <code>false</code> if <code>bar</code> is a function. In most cases, this is the desired behavior, but in situations where you want to also return <code>true</code> for functions, you could amend the above solution to be:
<pre><code class=" hljs coffeescript">console.log((bar !== null) &amp;&amp; ((typeof bar === "object") || (typeof bar === "function"))); </code></pre>
Second, the above solution will return <code>true</code> if <code>bar</code> is an array (e.g., if <code>var bar = [];</code>). In most cases, this is the desired behavior, since arrays are indeed objects, but in situations where you want to also <code>false</code> for arrays, you could amend the above solution to be:
<pre><code class=" hljs coffeescript">console.log((bar !== null) &amp;&amp; (typeof bar === "object") &amp;&amp; (toString.call(bar) !== "[object Array]")); </code></pre>
Or, if you&rsquo;re using jQuery:
<pre><code class=" hljs javascript">console.log((bar !== null) &amp;&amp; (typeof bar === "object") &amp;&amp; (! $.isArray(bar)));</code></pre>
<h1 style="text-align: center;">---</h1>
<h1 style="text-align: center;">scrap</h1>
&nbsp;
typeof a(number) !== 'undefined' True typeof b(number) !== 'undefined) true &nbsp;

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