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copied!<p>If you don't provide an explicit <code>return</code>, the return value is <code>undefined</code>. Your second function <em>always</em> returns <code>undefined</code>, in other words. Because the function always returns <code>undefined</code>, there'll never be anything included in the JSON stringified result, so the overall effect is that you get <code>undefined</code>.</p> <p>It's sort-of an anti-pattern to do this:</p> <pre><code>if (something) { return whatever; } else { return somethingElse; } </code></pre> <p>Your first function would be more idiomatic as:</p> <pre><code>str = JSON.stringify(o,function(k,v) { return Object.isSealed(this) ? undefined : v; }); </code></pre> <p><em>edit</em> — note that <code>JSON.stringify()</code> calls your replacer function <em>first</em> with an empty key, and "v" equal to <strike><code>this</code></strike> a new <em>wrapper</em> object containing the object to be stringified (and note that <em>that</em> object is <em>not</em> sealed). That is, before checking each property of an object to be stringified, it asks the replacer function whether it should work on the object as a whole. Because your function always returns <code>undefined</code>, the stringify function thinks that there's absolutely nothing to be done. It can't return an empty object because your replacer function immediately told it "don't include this in the result".</p> <p>Also note that the caller of your function cannot determine whether your function explicitly returned <code>undefined</code> or if it returned <code>undefined</code> because it didn't return anything, in case that's not clear. Those two cases look exactly the same to the caller.</p>

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