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POUsing Logical AND in Javascript Variable Declarations
 

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  1. POUsing Logical AND in Javascript Variable Declarations
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    <p>I am interested in the practical application of declaring variables using <code>&amp;&amp;</code> like this:</p> <pre><code>var x = undefined &amp;&amp; 4; // Evaluate to the first falsey value // or else the last value. eval(x); // undefined </code></pre> <p>I understand how the value is evaluated (see <a href="https://stackoverflow.com/a/7602012/1351736">this SO answer</a>). I also understand its sister <code>||</code> (see <a href="https://stackoverflow.com/a/476445/1351736">here</a> for a great description) and why it would be useful to declare a variable with the following expression:</p> <pre><code>// Some other variable var y; var x = y || 4; // Evaluate to the first truthy value // or else the last value. </code></pre> <p>Practically: Use the first value unless that first value is falsey; if so, use the last value. We can demonstrate this characteristic of <code>||</code> in the browser console:</p> <pre><code>&gt; null || 4 4 &gt; 4 || null 4 &gt; null || undefined undefined &gt; undefined || null null &gt; true || 4 true &gt; 4 || true 4 </code></pre> <p>As for <code>&amp;&amp;</code>:</p> <pre><code>&gt; null &amp;&amp; 4 null &gt; 4 &amp;&amp; null null &gt; null &amp;&amp; undefined null &gt; undefined &amp;&amp; null undefined &gt; true &amp;&amp; 4 4 &gt; 4 &amp;&amp; true true </code></pre> <p>Should we take this to mean: Use the first value unless that first value is truthy; if so, use the last value?</p> <p>I'm interested in using coding shortcuts to minimize the use of conditional statements, and I wonder if I might be able to use this one somehow.</p> <p>I found an example of this coding method in <a href="https://github.com/jquery/jquery/blob/451d0c37d9297598743b1372bf623d4b7d38e951/src/core.js#L472" rel="nofollow noreferrer">line 472</a> of the jQuery core source:</p> <pre><code>scripts = !keepScripts &amp;&amp; []; </code></pre> <p>So the question is this: Can anyone describe a good context for using <code>&amp;&amp;</code> in a javascript variable declaration? Do you consider it to be bad practice?</p> <p>Thanks!</p>
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    1. COI'd personally recommend against it: It's really not very readable. Without knowing about this behaviour, I'd expect something like `!keepScripts && []` to evaluate to `false`, which could lead to some difficult to track down bugs.
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    2. COI would write normal conditional statements and then use advanced minifiers, such as the [Google Closure Compiler](https://developers.google.com/closure/compiler/) to minify the code.
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