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POExtending the native JavaScript Error constructor
 

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  1. POExtending the native JavaScript Error constructor
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    <p>I tried to extend the JavaScript error properties through the extension of the prototype of the Error constructor:</p> <pre><code>&lt;script type="text/javascript"&gt; // extending the Error properties to all the ones available in the various browsers: Error.prototype = { name: null, // (all browsers); the name of the error message: null, // (all browsers); the error message as a string description: null, // (Internet Explorer); description of the error fileName: null, // (Firefox); the name of the file where the error occurred lineNumber: null, // (Firefox); the number of line where the error occurred columnNumber: null, // (Firefox); the number of column where the error occurred number: null, // (Internet Explorer); the error code as a number stack: null // (Firefox, Chrome); detailed information about the location where the error exactly occurred }; function log(error) { var errors = []; for (var prop in error) { errors.push(prop + ': ' + error[prop]); } alert(errors.join('\n')); } &lt;/script&gt; </code></pre> <p>Then I test the log function:</p> <pre><code>&lt;script type="text/javascript&gt; try { var a = b; // b is undefined! } catch(error) { log(error); } &lt;/script&gt; </code></pre> <p>The result is that the error object only shows some properties (e.g. on Firefox <code>fileName</code>, <code>lineNumber</code> and <code>columnNumber</code>) like if it has not been extended.</p> <p>But the most strange thing is that the <code>for...in</code> cycle seems <strong>unable to walk all the error object properties</strong>: trying to alert the standard property <code>error.message</code> normally returns a message.</p> <p>So the results of my test are:</p> <ol> <li>the Error constructor is not extensible through its prototype, as other native constructors are;</li> <li>the <code>for...in</code> loop is not able to walk the properties of an error object.</li> </ol> <p>Am I right?<br /> Are there some interesting evidences/resources you may suggest to know more about it?</p>
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    1. USEmanuele Del Grande
    1. USEmanuele Del Grande
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    1. CO`Error` is a host object and therefore does not have to behave like "normal" JavaScript objects.
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      1. USFelix Kling
    2. COMmh, interesting, I did not know about host objects. I am learning through something about through Stackoverflow threads. As for now, I understood they are special objects with special properties and behaviours, such as DOM objects, am I right?
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      1. USEmanuele Del Grande
    3. COSee http://es5.github.com/#x4.3.8. If you search for `host object` on this site you'll find some interesting statements about them. *edit*: They are basically objects supplied the environment and not part of the specification (just like DOM objects, yes). But the problem is probably just that `Error.prototype` is not writable (at least in Firefox). `var obj = {foo: 'bar'}; Error.prototype = obj; Error.prototype == obj;` results in `false`.
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