Docs Help

Terms, Icons, and Labels

Many classes have shortcut names used when creating (instantiating) a class with a configuration object. The shortcut name is referred to as an alias (or xtype if the class extends Ext.Component). The alias/xtype is listed next to the class name of applicable classes for quick reference.

Access Levels

Framework classes or their members may be specified as private or protected. Else, the class / member is public. Public, protected, and private are access descriptors used to convey how and when the class or class member should be used.

Member Types

Member Syntax

Below is an example class member that we can disect to show the syntax of a class member (the lookupComponent method as viewed from the Ext.button.Button class in this case).

lookupComponent ( item ) : Ext.Component

Called when a raw config object is added to this container either during initialization of the items config, or when new items are added), or {@link #insert inserted.

This method converts the passed object into an instanced child component.

This may be overridden in subclasses when special processing needs to be applied to child creation.


item :  Object

The config object being added.


The component to be added.

Let's look at each part of the member row:

Member Flags

The API documentation uses a number of flags to further commnicate the class member's function and intent. The label may be represented by a text label, an abbreviation, or an icon.

Class Icons

- Indicates a framework class

- A singleton framework class. *See the singleton flag for more information

- A component-type framework class (any class within the Ext JS framework that extends Ext.Component)

- Indicates that the class, member, or guide is new in the currently viewed version

Member Icons

- Indicates a class member of type config

- Indicates a class member of type property

- Indicates a class member of type method

- Indicates a class member of type event

- Indicates a class member of type theme variable

- Indicates a class member of type theme mixin

- Indicates that the class, member, or guide is new in the currently viewed version

Class Member Quick-Nav Menu

Just below the class name on an API doc page is a row of buttons corresponding to the types of members owned by the current class. Each button shows a count of members by type (this count is updated as filters are applied). Clicking the button will navigate you to that member section. Hovering over the member-type button will reveal a popup menu of all members of that type for quick navigation.

Getter and Setter Methods

Getting and setter methods that correlate to a class config option will show up in the methods section as well as in the configs section of both the API doc and the member-type menus just beneath the config they work with. The getter and setter method documentation will be found in the config row for easy reference.

History Bar

Your page history is kept in localstorage and displayed (using the available real estate) just below the top title bar. By default, the only search results shown are the pages matching the product / version you're currently viewing. You can expand what is displayed by clicking on the button on the right-hand side of the history bar and choosing the "All" radio option. This will show all recent pages in the history bar for all products / versions.

Within the history config menu you will also see a listing of your recent page visits. The results are filtered by the "Current Product / Version" and "All" radio options. Clicking on the button will clear the history bar as well as the history kept in local storage.

If "All" is selected in the history config menu the checkbox option for "Show product details in the history bar" will be enabled. When checked, the product/version for each historic page will show alongside the page name in the history bar. Hovering the cursor over the page names in the history bar will also show the product/version as a tooltip.

Search and Filters

Both API docs and guides can be searched for using the search field at the top of the page.

On API doc pages there is also a filter input field that filters the member rows using the filter string. In addition to filtering by string you can filter the class members by access level and inheritance. This is done using the checkboxes at the top of the page.

The checkbox at the bottom of the API class navigation tree filters the class list to include or exclude private classes.

Clicking on an empty search field will show your last 10 searches for quick navigation.

API Doc Class Metadata

Each API doc page (with the exception of Javascript primitives pages) has a menu view of metadata relating to that class. This metadata view will have one or more of the following:

Expanding and Collapsing Examples and Class Members

Runnable examples (Fiddles) are expanded on a page by default. You can collapse and expand example code blocks individually using the arrow on the top-left of the code block. You can also toggle the collapse state of all examples using the toggle button on the top-right of the page. The toggle-all state will be remembered between page loads.

Class members are collapsed on a page by default. You can expand and collapse members using the arrow icon on the left of the member row or globally using the expand / collapse all toggle button top-right.

Desktop -vs- Mobile View

Viewing the docs on narrower screens or browsers will result in a view optimized for a smaller form factor. The primary differences between the desktop and "mobile" view are:

Viewing the Class Source

The class source can be viewed by clicking on the class name at the top of an API doc page. The source for class members can be viewed by clicking on the "view source" link on the right-hand side of the member row.

Ext JS 5.1.0


Identifying Memory Leaks

The term "memory leak" is used in many contexts. It is often used to describe memory growth. Wikipedia defines a "memory leak" as such:

"When a computer program incorrectly manages memory allocations".

This is a reasonable definition, but it is a bit vague.

Our Definition

For the purposes of this guide, a memory leak is defined as:

When memory usage grows without limit after repeating a portion of code. The code must be repeated "to exhaustion" (enough that it would be necessary to reclaim memory) and the code must also ensure that reasonable language / framework cleanup has been performed.

That's a bit of mouthful, so let's break down the important bits of this definition:

Language/Framework Cleanup

Depending on the environment in which the program runs, there are typically rules regarding actions you should take to indicate that you are finished with a certain piece of allocated memory. In Ext JS, this is typically the destroy method, which generally cleans up DOM elements and unbinds listeners.

In C#, the recommended pattern is the IDisposable interface. Regardless of the platform, these conventions must be followed to allow the platform to release allocated resources. If cleanup procedures are not followed, memory leaks will result because it is not possible to automatically infer when resources are no longer needed.

Repeating to exhaustion

Let's assume there is a development machine with 64Gb of free memory. A section of code is run 5 times. By inspection, it's noted that after each run, memory usage increases 1Mb each time and is never reclaimed.

This observation is not really indicative of a problem. The program is only using a tiny fraction of available memory. If the code section is repeated 50,000 times and still none of the memory is reclaimed, this would be a different result. The underlying system needs to be sufficiently stressed so that it is forced to reclaim memory.

Usage Grows Unboundedly

This is probably the most subtle, yet most important part of the definition. In many cases, calling destroy or other cleanup may not free all allocated resources. In Ext JS this is typically observed in its caches.

For example, the Ext.ComponentQuery class is used to search components based on a string selector. Internally, this string selector is transformed into a function that can be executed on the candidate components. Constructing this function is expensive and, oftentimes, the same query is run multiple times. Due to this re-use, the generated function is kept in memory. The crucial point here is that the caching mechanism is bounded.

The cache is an LRU (Least Recently Used) cache. The LRU keeps track of accesses to items in the collection. When an item is accessed, it is pulled to the front. The LRU cache also has a maximum size. When adding an item exceeds the maximum size, the least recently used item is evicted from the cache. Once the maximum limit is reached, the cache normalizes. Things of this nature remaining in memory is not problematic. It only becomes an issue when resources are retained without limit.

Abstraction and Garbage Collection

A developer using Ext JS is far-removed from real memory management. Worse still, tools such as Window Task Manager or Mac Activity Monitor do not provide accurate depictions of memory consumption. To better understand how far removed the cause and effect relationships are, it is important to evaluate the layers of memory management.


  • The developer requests resources from the framework (for example, creating a component).
  • The framework requests resources from the JavaScript engine (often using operator new or createElement, etc.).
  • The JavaScript engine requests resources from the underlying process memory manager (typically a C++ memory allocation).
  • The underlying memory manager requests resources from the operating system. This is the memory growth we can observe in Task Manager and Activity Monitor.


  1. The developer calls destroy on an Ext JS component or other resource.
  2. The destroy method of the Ext JS component calls other cleanup methods, sets various internal references to null, etc..
  3. The JavaScript garbage collector later decides when to sweep over the heap and reclaim memory. This is often deferred until new memory is requested and there is "insufficient" free memory. The memory manager may simply decide to grow the heap again instead of collecting garbage since growing the heap is often cheaper, especially early in the life of the application.
  4. Once the JavaScript memory manager decides to collect garbage, it has to decide if the reclaimed memory should be retained as free memory for its future use or returned to the underlying process heap.
  5. Depending on the underlying memory manager used by the JavaScript memory manager (typically a C++ memory manager) the free memory may be kept for future use by that process or returned to the operating system. Only if and when we reach this point do we see any updates in Task Manager / Activity Monitor.

Given the above, it is clear that the JavaScript developer has little control over the big picture in regards to memory management. There are many moving parts and the real memory management is a very small cog.

For the purpose of this guide, we will not discuss these layers further. It is sufficient to say that the JavaScript heap and its garbage collector perform the actions they deem appropriate and it is not possible to force them to behave in a particular fashion. The best we can do is ensure that references are not being held by user code or by the framework.

Ultimately, inspecting memory usage with common OS monitoring tools and observing increases in not necessarily indicative of a "memory leak".

Detecting Leaks

Application Level Leaks

When applications fail to cleanup framework resources, this can cause objects to accumulate in several collections maintained by the framework. While the exact details of these are version-specific, some places to check are:

Framework Level Leaks

While every effort is made to cleanup resources internal to the framework, there is always room for mistakes. Historically, the most common issues have come from leaking DOM elements. If you suspect this is the case, the sIEve tool provides excellent leak detection in Internet Explorer.

Note: We highly recommended that you address all application-level leaks before looking at things on this lower level.

Common Code Leak Patterns And Solutions

The following code snippets and descriptions will highlight various ways that memory is abused in a way that may cause problems.

Preventing Base Class Cleanup

In an effort to clean up resources in derived classes, base class cleanup may be accidentally bypassed.

For example:

Ext.define('', {
    extend: 'Ext.button.Button',
    onDestroy: function () {
        // do some cleanup

Solution: Be sure to call callParent(), which allows the base class to perform its cleanup.

Not removing DOM listeners

An event is attached to an element. The elements is overwritten by changing the innerHTML. However, this event handler will remain in memory.'click', doSomething);

someElement.parentNode.innerHTML = '';

Solution: Keep a reference to important elements and call their destroy method when they are no longer needed.

Keeping references to objects

An instance of a class is created that uses lots of memory. The class is destroyed, but a reference remains on an existing object.

Ext.define('MyClass', {

    constructor: function() { = new SomeLargeObject();

    destroy: function() {;

this.o = new MyClass();

// `this` still has a reference to `o` and `o` has a reference to `foo`.

Solution: Set references to null to ensure memory can be reclaimed. In this case, = null in destroy as well as this.o = null after calling destroy.

Keeping References in Closures

This situation is more subtle, but very similar to the above. The closure holds a reference to a large object that can't be reclaimed while the closure is still being referenced.

function runAsync(val) {
    var o = new SomeLargeObject();
    var x = 42;

    // other things

    return function() {
        return x;  // o is in closure scope but not needed

var f = runAsync(1);

The above often occurs because the large object was present in the outer scope and not needed by the inner function. These sorts of things are easy to miss, but can negatively affect memory usage.

Solution: Use Ext.Function.bind() or the standard JavaScript Function bind to create safe closures for functions declared outside such functions.

function fn (x) {
    return x;

function runAsync(val) {
    var o = new SomeLargeObject();
    var x = 42;

    // other things

    return Ext.Function.bind(fn, null, [x]); // o is not captured

var f = runAsync(1);

Continually creating instances with side effects

Creating some objects can have side effects (for example, creating DOM elements). If these are being created without being destroyed, they can leak memory.

    xtype: 'treepanel',
    listeners: {
        itemclick: function(view, record, item, index, e) {

            // Always creating and rendering a new menu
                items: [record.get('name')]

Solution: Capture a reference to the menu and call the destroy method on it when it is no longer needed.

Clearing any registration in a cache

It is important to remove all references to an object. Setting a local reference to null is not enough. If some global singleton cache is holding a reference, that reference will be held for the lifetime of the application.

var o = new SomeLargeObject();

// Destroy and null the reference. someCache still has a reference
o = null;

Solution: Be sure to remove objects from any caches to which it has been added in addition to calling destroy.


Taking control of your application's memory management can be a simple task. Keep your application above reproach by destroying your unused components, nullifying unused references, and using callParent(). Following these suggestions will ensure that your application is running smoothly and does not use resources irresponsibly.

Ext JS 5.1.0

Ext JS
Sencha Test
Sencha Themer
IDE Plugins
Sencha Inspector
Sencha Fiddle
Offline Documentation

Sencha Test

2.0.0 EA 1.0.3



Sencha Themer

1.1.0 EA 1.0.2


5.x EA 4.x 3.x

IDE Plugins

IDE Plugins

Sencha Inspector

Sencha Inspector

Sencha Fiddle

Sencha Fiddle

Offline Documentation

Offline Documentation