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copied!<p>The difference is that <code>functionOne</code> is a function expression and so only defined when that line is reached, whereas <code>functionTwo</code> is a function declaration and is defined as soon as its surrounding function or script is executed (due to <a href="http://adripofjavascript.com/blog/drips/variable-and-function-hoisting.html" rel="noreferrer">hoisting</a>). </p> <p>For example, a function expression:</p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>// TypeError: functionOne is not a function functionOne(); var functionOne = function() { console.log("Hello!"); };</code></pre> </div> </div> </p> <p>And, a function declaration: </p> <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false"> <div class="snippet-code"> <pre class="snippet-code-js lang-js prettyprint-override"><code>// Outputs: "Hello!" functionTwo(); function functionTwo() { console.log("Hello!"); }</code></pre> </div> </div> </p> <p>This also means you can't conditionally define functions using function declarations:</p> <pre><code>if (test) { // Error or misbehavior function functionThree() { doSomething(); } } </code></pre> <p>The above actually defines <code>functionThree</code> irrespective of <code>test</code>'s value — unless <code>use strict</code> is in effect, in which case it simply raises an error.</p>

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