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    <p>Ignore the error. The "in" operator is clearly defined in <a href="http://ecma-international.org/ecma-262/5.1/" rel="noreferrer">ECMA 262-5.1 / June 2011</a> sec-11.8.7</p> <p>There appear to be only three outcomes for <code>'someProperty' in anObject</code> which is one of: <code>true</code>, <code>false</code>, or a <code>TypeError</code> exception is thrown. When it evaluates to <code>true</code>, then there is definitely <code>'someProperty' in anObject</code>. When it evaluates to <code>false</code> there is definitely not <code>'someProperty' in anObject</code>. When a <code>TypeError</code> is thrown there is definitely not <code>'someProperty' in anObject</code> because <code>anObject</code> is either null or it isn't an object at all. This all seems very clear to me. When I want to know if an object has a property and, I don't care if that property is the object's own property or being inherited and, I don't care what the value of the property is, then I simply look for <code>'someProperty' in anObject</code>.</p> <h3> Warning </h3> <p><i>Caution: some would have you check for <code>anObject.someProperty !== undefined</code> but that isn't really checking whether or not the object has the property. What it's doing is checking whether the object has the property <b>AND</b> that the value of that property is <b>NOT</b> undefined. Some would have you check for <code>anObject.hasOwnProperty('someProperty');</code> but that will only tell you if the object has that property <b>AND</b> has <b>NOT</b> inherited it somehow. Don't believe me? Try the following:</i></p> <pre><code>console.log(document.body.tagName); // BODY console.log(document.body.hasOwnProperty('tagName')); // false, it's inherited console.log('tagName' in document.body); // true, it does have the property document.body.wobbles = undefined; // new property added to document.body console.log('wobbles' in document.body); // true, it does have the property console.log(document.body.wobbles !== undefined); // false, the value really IS undefined </code></pre> <p>I've written an article about the "in" operator that goes into more detail. If you want to read it it's at: <a href="http://matthewkastor.blogspot.com/2012/09/Unexpected--in---Compare-with-undefined--or-use-the-hasOwnProperty-method-instead.html" rel="noreferrer"><a href="http://matthewkastor.blogspot.com/2012/09/Unexpected--in---Compare-with-undefined--or-use-the-hasOwnProperty-method-instead.html" rel="noreferrer">http://matthewkastor.blogspot.com/2012/09/Unexpected--in---Compare-with-undefined--or-use-the-hasOwnProperty-method-instead.html</a></a> The bottom line is that you should just ignore this error and wrap things up in a try catch block if the object might be null or not an object.</p> <pre><code>function hasProperty(prop, obj) { try { return prop in obj; } catch(e) { return e; } } </code></pre>
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    1. POjslint error: Unexpected 'in'. Compare with undefined, or use the hasOwnProperty
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    1. COThis is a great discussion of reasons why you should use `in`. But do you have any idea why JSLint thinks you should not use it? If you don't know why jslint makes recommends against it, then you don't know if you're addressing its concerns.
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    2. CO@user577888 Yes, I know why JSLint gives the notice and that the notice can not be suppressed. The above post explains the usage of `in` and why it can safely be ignored. To put it simply, the warning comes up for performance reasons. It makes no sense to search the entire prototype chain when you're expecting the property to be directly assigned to the object in question. Using `in` without the `hasOwnProperty` guard isn't really an error and there aren't any harmful side effects in doing so.
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