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copied!<h2>What is a ReferenceError?</h2> <p>As defined by ECMAScript 5, a <a href="http://es5.github.com/#x15.11.6.3" rel="noreferrer"><code>ReferenceError</code></a> indicates that an invalid reference has been detected. That doesn't say much by itself, so let's dig a little deeper.</p> <p>Leaving aside strict mode, a <code>ReferenceError</code> occurs when the scripting engine is instructed to get the value of a reference that it <a href="http://es5.github.com/#IsUnresolvableReference" rel="noreferrer">cannot resolve the <em>base value</em></a> for:</p> <blockquote> <p>A Reference is a resolved name binding. A Reference consists of three components, the base value, the referenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value of undefined indicates that the reference could not be resolved to a binding. The referenced name is a String.</p> </blockquote> <p>When we are referencing a property, the base value is the object whose property we are referencing. When we are referencing a variable, the base value is unique for each execution context and it's called an <em>environment record</em>. When we reference something that is neither a property of the base object value nor a variable of the base environment record value, a <code>ReferenceError</code> occurs.</p> <p>Consider what happens when you type <code>foo</code> in the console when no such variable exists: you get a <code>ReferenceError</code> because the <em>base value</em> is <em>not resolvable</em>. However, if you do <code>var foo; foo.bar</code> then you get a <code>TypeError</code> instead of a <code>ReferenceError</code> -- a subtle perhaps but very significant difference. This is because the <em>base value</em> was successfully resolved; however, it was of type <code>undefined</code>, and <code>undefined</code> does not have a property <code>bar</code>.</p> <h2>Guarding against ReferenceError</h2> <p>From the above it follows that to catch a ReferenceError before it occurs you have to make sure that the base value is resolvable. So if you want to check if <code>foo</code> is resolvable, do</p> <pre><code>if(this.foo) //... </code></pre> <p>In the global context, <code>this</code> equals the <code>window</code> object so doing <code>if (window.foo)</code> is equivalent. In other execution contexts it does not make as much sense to use such a check because by definition it's an execution context your own code has created -- so you should be aware of which variables exist and which do not.</p>

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