Editor’s note: This article was updated by our editorial team on July 19, It has been modified to include recent sources and to align with our current editorial standards.
Consider this example code snippet:
Executing the above code results in the following error:
Why? It’s all about context. The reason you get the above error is because, when you invokeyou are actually invoking. As a result, the anonymous function being passed to is being defined in the context of the object, which has no method.
A traditional, old-browser-compliant solution is to simply save your startup error windows vista to in a variable that can then be inherited by the closure; e.g.:
Alternatively, in newer browsers, you can use the method to pass in the proper reference:
If you guess that the call would either output or throw an error, error 010 functuion or declaration, you guessed incorrectly. Believe it or not, it will output. Why?
If that’s news to you, it’s worth taking the time to read up on scopes, prototypes, and more.
Memory Leak Example 1: Dangling References to Defunct Objects
Consider the following code:
If you run the above code and monitor memory usage, you’ll find that you’ve got a significant memory leak, leaking a full megabyte per second! And even a manual Error 010 functuion or declaration Collector (GC) doesn’t help. So it looks like we are leaking every error 010 functuion or declaration is called. But why?
Let’s examine things in more detail:
Each object contains its own 1MB object. Every second, when we callit holds on to a reference to the prior object in. But we still wouldn’t think this would be a problem, since each time through, the previously referenced would be dereferenced (when is reset via ). And moreover, is only referenced in the main body of and in the function which is, error 010 functuion or declaration, in fact, never used.
So again we’re left wondering why there is a memory leak here.
Memory Leak Example 2: Circular References
Consider this code fragment:
Here, has a closure that keeps a reference to (via ). By also assigning tothe circular reference is created; i.e.: → → → → …
Interestingly, even if is removed from the DOM, the circular self-reference above would prevent and from being collected, and hence, a memory leak.
Avoiding Memory Leaks: The Essentials
The following objects are assumed to be reachable and are known as “roots”:
- Objects referenced from anywhere in the current call stack (that is, all local variables and parameters in the functions currently being invoked, and all the variables in the closure scope)
- All global variables
Objects are kept in memory at least as long as they are accessible from any of the roots through a reference, or a chain of references.
There is a Garbage Collector in the browser that cleans memory occupied by unreachable objects; in other words, error 010 functuion or declaration, objects will be removed from memory if and only if the GC believes that they are unreachable. Unfortunately, it’s fairly easy to end up with defunct “zombie” objects that are no longer in use but that the GC still thinks are “reachable.”
And completely as a sidepoint—but since we’re talking about type coercion and comparisons—it’s worth mentioning that comparing with anything (even !) will always return. You therefore cannot use the equality operators (, ) to determine whether a value is or not. Instead, use the built-in global function:
A common example is code that adds a series of DOM Elements one at a time. Adding a DOM element is an expensive operation. Code that adds multiple DOM elements consecutively is inefficient and likely not to work well.
One effective alternative when multiple DOM elements need to be added is to use document fragments instead, thereby improving efficiency and performance.
In addition to the inherently improved efficiency of this approach, creating attached DOM elements is expensive, whereas creating and modifying them while detached and then attaching them yields much better performance.
Consider this code:
Based on the above code, if there were 10 input elements, clicking any of them would display “This is element #10”! This is because, by the time is invoked for any of the elements, the above loop will have completed and the error 010 functuion or declaration of will already be 10 (for all of them).
In this revised version of the code, is immediately executed each time we pass through the loop, each time receiving the then-current value of and binding it to a scoped variable. The error 010 functuion or declaration function dj catalog error loading feed data the inner function (which also uses this scoped variable) and the element’s is set to that inner function. This ensures that each receives and uses the proper value (via the scoped variable).
Here’s a simple example. Consider this code:
Seems error 010 functuion or declaration straightforward. If you provide a name, use it, otherwise set the name to ‘default’. For instance:
But what if we were to do this:
We’d then get:
But wouldn’t it be nicer for this to revert to ‘default’? This can easily be done, if we modify the original code to leverage prototypal inheritance, as follows:
With this version, inherits the property from its object, where it is set (by default) to. Thus, if the constructor is called without a name, the name will default to. And similarly, if the property is removed from an instance ofthe prototype chain will then be searched and the property will be retrieved from the object where its value is still. So now we get:
Let’s define a simple object, and create an instance of it, as follows:
Now, for convenience, let’s create a reference to the method, presumably so we can access it merely by rather than the longer :
And just to be sure everything looks copacetic, let’s print out the value of our new variable:
Okay, cool. Looks fine.
But now, look at the difference when we invoke vs. our convenience reference :
What went wrong? When we did the assignmentthe new variable was being defined in the global namespace. As a result, its value of isnot the instance of !
Thus, if we really need to create a reference to an existing method of an object, we need to be sure to do it within that object’s namespace, to preserve the value of. One way of doing this would be as follows:
The alternative to passing a string as the first argument to these methods is to instead pass in a function. Let’s take a look at an example.
Here, then, would be a fairly typical use of andpassing a string as the first parameter:
The better choice would be to pass in a function as the initial argument; e.g.:
While, admittedly, failing to use strict mode is not a “mistake” per se, its use is increasingly being encouraged and its omission is increasingly becoming considered bad form.
Here are some key benefits of strict mode:
- Eliminates coercion. Without strict mode, a reference to a value of null or undefined is automatically coerced to the global. This can cause many frustrating bugs. In strict mode, peoneer error 30 a value of null or undefined throws an error.
- Disallows duplicate property names or parameter values. Strict mode throws an error when it detects a duplicate named property in an object (e.g., ) or a duplicate named argument for a function (e.g., ), thereby catching what is almost certainly a bug in your code that you might otherwise have wasted lots of time tracking down.
- Throws error on invalid use of . The operator (used to remove properties from objects) cannot be used on nonconfigurable properties of the object. Nonstrict code will fail silently when an attempt is made to delete a nonconfigurable property, whereas strict mode will throw an error in such a case.
Further Reading on the Toptal Engineering Blog: