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copied!<p>The <code>call</code> method sets the <code>this</code> value of the invoked function to the object passed as first argument, in your example, you are executing the <code>Object.prototype.toString</code> method on an Array object.</p> <p>Array objects, have their own <code>toString</code> method (<code>Array.prototype.toString</code>) that shadows the one from <code>Object.prototype</code>, if you call <code>[].toString();</code> the method on the <code>Array.prototype</code> will be invoked.</p> <p>For example:</p> <pre><code>function test() { alert(this); } test.call("Hello"); // alerts "Hello" </code></pre> <p>Another example:</p> <pre><code>var alice = { firstName: 'Alice', lastName: 'Foo', getName: function () { return this.firstName + ' ' + this.lastName; } }; var bob = { firstName: 'Bob', lastName: 'Bar', }; alice.getName.call(bob); // "Bob Bar" </code></pre> <p>In the above example, we use the Alice's <code>getName</code> method on the Bob's object, the <code>this</code> value points to <code>bob</code>, so the method works just as if it were defined on the second object.</p> <p>Now let's talk about the <code>Object.prototype.toString</code> method. All native objects in JavaScript contain an <em>internal property</em> called <code>[[Class]]</code> this property contains a string value that represents the specification defined <em>classification</em> of an object, the possible values for <em>native</em> objects are:</p> <ul> <li><code>"Object"</code></li> <li><code>"Array"</code></li> <li><code>"Function"</code></li> <li><code>"Date"</code></li> <li><code>"RegExp"</code></li> <li><code>"String"</code></li> <li><code>"Number"</code></li> <li><code>"Boolean"</code></li> <li><code>"Error"</code> for error objects such as instances of <code>ReferenceError</code>, <code>TypeError</code>, <code>SyntaxError</code>, <code>Error</code>, etc</li> <li><code>"Math"</code> for the global <code>Math</code> object</li> <li><code>"JSON"</code> for the global JSON object defined on the ECMAScript 5th Ed. spec.</li> <li><code>"Arguments"</code> for the <code>arguments</code> object (also introduced on the ES5 spec.)</li> <li><code>"null"</code> (introduced just a couple of days ago in the <a href="http://wiki.ecmascript.org/lib/exe/fetch.php?id=es3.1%3Aes3.1_proposal_working_draft&cache=cache&media=resources:es5_errata_7-1-10.pdf">ES5 errata</a>)</li> <li><code>"undefined"</code></li> </ul> <p>As I've said before that property is <em>internal</em>, there is no way to change it, the specification doesn't provide any operator or built-in function to do it, and the <em>only</em> way you can access its value is through the <code>Object.prototype.toString</code> method.</p> <p>This method returns a string formed by:</p> <pre><code>"[object " + this.[[Class]] + "]" </code></pre> <p><sub>Only for expository purposes because <code>[[Class]]</code> cannot be accessed directly.</sub></p> <p>For example:</p> <pre><code>Object.prototype.toString.call([]); // "[object Array]" Object.prototype.toString.call(/foo/); // "[object RegExp]" Object.prototype.toString.call({}); // "[object Object]" Object.prototype.toString.call(new Date); // "[object Date]" // etc... </code></pre> <p>This is really useful to detect the <em>kind</em> of an object in a safe way, for detecting array objects, it's the most widely used technique:</p> <pre><code>function isArray(obj) { return Object.prototype.toString.call(obj) == '[object Array]'; } </code></pre> <p>It might be tempting to use the <code>instanceof</code> operator, but that way will lead to problems if you work on <em>cross-frame</em> environments, because an array object created on one frame, will not be <code>instanceof</code> the <code>Array</code> constructor of another.</p> <p>The above method will work without any problems, because the object will contain the value of its <code>[[Class]]</code> internal property intact.</p> <p>See also: </p> <ul> <li><a href="http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/"><code>instanceof</code> considered harmful (or how to write a robust <code>isArray</code>)</a></li> <li><a href="http://ecma262-5.com/ELS5_Section_15.htm#Section_15.2.4.2"><code>Object.prototype.toString</code></a></li> <li><a href="http://ecma262-5.com/ELS5_Section_8.htm#Section_8.6.2">Object Internal Properties and Methods</a></li> </ul>

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