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    <p>My understanding is that this is actually very simple:</p> <ul> <li>Javascript is <em>always</em> pass by value, but when a variable refers to an object (including arrays), the "value" is a reference to the object. </li> <li>Changing the value of a variable <em>never</em> changes the underlying primitive or object, it just points the variable to a new primitive or object.</li> <li>However, changing a <em>property</em> of an object referenced by a variable does change the underlying object.</li> </ul> <p>So, to work through some of your examples:</p> <pre><code>function f(a,b,c) { // Argument a is re-assigned to a new value. // The object or primitive referenced by the original a is unchanged. a = 3; // Calling b.push changes its properties - it adds // a new property b[b.length] with the value "foo". // So the object referenced by b has been changed. b.push("foo"); // The "first" property of argument c has been changed. // So the object referenced by c has been changed (unless c is a primitive) c.first = false; } var x = 4; var y = ["eeny", "miny", "mo"]; var z = {first: true}; f(x,y,z); console.log(x, y, z.first); // 4, ["eeny", "miny", "mo", "foo"], false </code></pre> <p>Example 2:</p> <pre><code>var a = ["1", "2", {foo:"bar"}]; var b = a[1]; // b is now "2"; var c = a[2]; // c now references {foo:"bar"} a[1] = "4"; // a is now ["1", "4", {foo:"bar"}]; b still has the value // it had at the time of assignment a[2] = "5"; // a is now ["1", "4", "5"]; c still has the value // it had at the time of assignment, i.e. a reference to // the object {foo:"bar"} console.log(b, c.foo); // "2" "bar" </code></pre>
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    1. POJavascript by reference vs. by value
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    1. COWhile technically true, I prefer to say JavaScript is [Pass By Object Sharing](http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing). It avoids such confusion and moves to a "high level" view.
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    2. CO@pst - That makes sense, but note the issues raised in the referenced Wikipedia page - most people don't use the term, and the (large) Java community calls this "pass-by-value". Still, I agree that it's a bit confusing.
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