Invalid Left-Hand Side In Assignment Ext Js 4


Terms, Icons, and LabelsAccess LevelsMember TypesMember SyntaxMember FlagsClass IconsMember IconsNavigation and FeaturesClass Member Quick-Nav MenuGetter and Setter MethodsHistory BarSearch and FiltersAPI Doc Class MetadataExpanding and Collapsing Examples and Class MembersDesktop -vs- Mobile ViewViewing the Class Source

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 (or 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 or . Else, the class / member is . , , and are access descriptors used to convey how and when the class or class member should be used.

  • Public classes and class members are available for use by any other class or application code and may be relied upon as a stable and persistent within major product versions. Public classes and members may safely be extended via a subclass.

  • Protected class members are stable members intended to be used by the owning class or its subclasses. Protected members may safely be extended via a subclass.

  • Private classes and class members are used internally by the framework and are not intended to be used by application developers. Private classes and members may change or be omitted from the framework at any time without notice and should not be relied upon in application logic.

Member Types

  • Config - The configuration options for a class.
  • Property - Set once a class is instantiated. *See Read Only below.
  • Method - Actions that can be performed by a class. Methods should be read as instance methods and can only be called from a instance of a given class. Static methods that can be called directly from the class itself will have a label next to the method name. *See Static below.
  • Event - Events are specific to the framework event system allowing for classes to programmatically raise an event to be handled by one or more event handler methods. DOM events, while handled by the framework event system, are not specifically described within the API documentation. *For DOM events refer to the event reference page from MDN.
  • Theme Variable - Variables used by the visual theme engine employed by the framework.
  • Theme Mixin - Functions used by the visual theme engine employed by the framework and may use values set in various Theme Variables.

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).

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

  • Expand/Collapse - On the left-hand size of the member row is a control used to expand and collapse each member row to show / hide member details.
  • Member Name - The name of the class member ( in this example)
  • Method Param - Any required or optional params used by a method (or passed to an event handler method) will be listed next to the method name within parenthesis ( in this example)
  • Return type - The class instance or javascript object returned by the method or property ( in this case). This may be omitted for methods that do not return anything other than or may display as multiple possible values separated by a forward slash signifying that what is returned may depend on the results of the method call (i.e. a method may return a Component if a get method calls is successful or if unsuccessful which would be displayed as ).
  • Flags - Any flags applicable to the member will be displayed next ( in this example - see the Flags section below)
  • Member Origin - On the right-hand side of the member row is the class where the member was initially described ( in this example). The source class will be displayed as a blue link if the member originates from the current class and gray if it is inherited from an ancestor or mixed-in class.
  • Member Source - On the right-hand side below the member origin class is a link to view the member's source ( in the example)
  • Params List - Each param for a class method will be listed using the same name found above in parenthesis, the type of class or object expected, and a description of the param ( in the example).
  • Returns - If a class returns a value other than a "Returns" section will note the type of class or object returned and a description ( in the example)
  • Since (not shown in the example) - Some members will show which version of the product the member was first introduced (i.e. - not pictured in the example) just after the member description
  • Default (not shown in the example) - Configs often show the default config value to be applied to a class instance if not overridden (i.e. )

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.

  • Required - Required config when instantiating a class
  • Bindable - The config has a setter which allows this config to be set via ViewModel binding
  • Read Only - The property may be read, but cannot be used to configure / re-configure a class instance at runtime
  • Singleton - Singleton classes are instantiated immediately once defined and may not be instantiated manually
  • Static - A static method or property is a method or property belonging to the class itself, not an instance of the class
  • Chainable - Refers to methods that return the class instance back when called.
    This enables chained method calls like:
  • Deprecated - A class or member that is scheduled for removal in a future framework version and is provided in the current version for backwards compatibility.
    Deprecated classes and members will have a message directing you to the preferred class / method going forward.
  • Removed - A removed class or member that exists in documentation only as a reference for users upgrading between framework versions
  • Template - A method defined within a base class designed to be overridden by subclasses
  • Abstract - A class or member may be be defined as abstract. Abstract classes and members establish a class structure and provide limited, if any, code. Class-specific code will be furnished via overrides in subclasses.
  • Preventable - Events marked preventable will not fire if is returned from an event handler

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

- Indicates a class member of type

- Indicates a class member of type

- Indicates a class member of type

- Indicates a class member of type

- Indicates a class member of type

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

Navigation and Features

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, inheritance, and read only. 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:

  • Alternate Name - One or more additional class name synonymns (in Ext JS 6.0.0 the class has an alternate class name of ). Alternate class names are commonly maintained for backward compatibility.
  • Hierarchy - The hierararchy view lists the inheritance chain of the current class up through its ancestor classes up to the root base class.
  • Mixins - A list of classes that are mixed into the current class
  • Inherited Mixins - A list of classes that are mixed into an ancestor of the current class
  • Requires - All classes required to be defined for the class to be instantiated
  • Uses - A list of classes potentially used by the class at some point in its lifecycle, but not necessarily requried for the class to initially be instantiated
  • Subclasses - Classes that extend the current class

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:

  • Global navigation will be located in a menu on the left-hand side accessible via the hamburger menu icon. The menu houses the following (on most pages):
    • The name of the current product (as a link to the product landing page)
    • The Sencha icon used to navigate back to the documentation home page
    • The product menu drop-down button
    • Tabs of navigation trees for the API docs and guides
  • Current context navigation and tools is located on the right-hand side accessible via the gear icon. The context menu houses teh following:
    • The global search input field
    • (API doc) A "Filters" tab with the member filter, expand / collapse all examples button, expand / collapse all member rows button, the access level filter checkboxes, and the counts of each member
    • (API doc) A "Related Classes" tab containing the menu of metadata related to the current class
    • (Guides) The table of contents for the guide

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.

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.

JavaScript Errors and How to Fix Them

JavaScript can be a nightmare to debug: Some errors it gives can be very difficult to understand at first, and the line numbers given aren’t always helpful either. Wouldn’t it be useful to have a list where you could look to find out what they mean and how to fix them? Here you go!

Below is a list of the strange errors in JavaScript. Different browsers can give you different messages for the same error, so there are several different examples where applicable.

How to read errors?

Before the list, let’s quickly look at the structure of an error message. Understanding the structure helps understand the errors, and you’ll have less trouble if you run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a function

The structure of the error is as follows:

  1. Uncaught TypeError: This part of the message is usually not very useful. Uncaught means the error was not caught in a statement, and is the error’s name.
  2. undefined is not a function: This is the message part. With error messages, you have to read them very literally. For example in this case it literally means that the code attempted to use like it was a function.

Other webkit-based browsers, like Safari, give errors in a similar format to Chrome. Errors from Firefox are similar, but do not always include the first part, and recent versions of Internet Explorer also give simpler errors than Chrome – but in this case, simpler does not always mean better.

Now onto the actual errors.

Uncaught TypeError: undefined is not a function

Related errors: number is not a function, object is not a function, string is not a function, Unhandled Error: ‘foo’ is not a function, Function Expected

Occurs when attempting to call a value like a function, where the value is not a function. For example:

var foo = undefined; foo();

This error typically occurs if you are trying to call a function in an object, but you typed the name wrong.

var x = document.getElementByID('foo');

Since object properties that don’t exist are by default, the above would result in this error.

The other variations such as “number is not a function” occur when attempting to call a number like it was a function.

How to fix this error: Ensure the function name is correct. With this error, the line number will usually point at the correct location.

Uncaught ReferenceError: Invalid left-hand side in assignment

Related errors: Uncaught exception: ReferenceError: Cannot assign to ‘functionCall()’, Uncaught exception: ReferenceError: Cannot assign to ‘this’

Caused by attempting to assign a value to something that cannot be assigned to.

The most common example of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a single equals instead of two. The message “left-hand side in assignment” is referring to the part on the left side of the equals sign, so like you can see in the above example, the left-hand side contains something you can’t assign to, leading to the error.

How to fix this error: Make sure you’re not attempting to assign values to function results or to the keyword.

Uncaught TypeError: Converting circular structure to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Not an acyclic Object, TypeError: cyclic object value, Circular reference in value argument not supported

Always caused by a circular reference in an object, which is then passed into .

var a = { }; var b = { a: a }; a.b = b; JSON.stringify(a);

Because both and in the above example have a reference to each other, the resulting object cannot be converted into JSON.

How to fix this error: Remove circular references like in the example from any objects you want to convert into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, but it wasn’t there. Typically caused by mismatched parentheses or brackets.

The token in this error can vary – it might say “Unexpected token ]” or “Expected {” etc.

How to fix this error: Sometimes the line number with this error doesn’t point to the correct place, making it difficult to fix.

  • An error with [ ] { } ( ) is usually caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will often point to something else than the problem character
  • Unexpected / is related to regular expressions. The line number for this will usually be correct.
  • Unexpected ; is usually caused by having a ; inside an object or array literal, or within the argument list of a function call. The line number will usually be correct for this case as well

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to fix this error: Ensure all strings have the correct closing quote.

Uncaught TypeError: Cannot read property ‘foo’ of null, Uncaught TypeError: Cannot read property ‘foo’ of undefined

Related errors: TypeError: someVal is null, Unable to get property ‘foo’ of undefined or null reference

Attempting to read or as if it was an object. For example:

var someVal = null; console.log(;

How to fix this error: Usually caused by typos. Check that the variables used near the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot set property ‘foo’ of null, Uncaught TypeError: Cannot set property ‘foo’ of undefined

Related errors: TypeError: someVal is undefined, Unable to set property ‘foo’ of undefined or null reference

Attempting to write or as if it was an object. For example:

var someVal = null; = 1;

How to fix this error: This too is usually caused by typos. Check the variable names near the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, too much recursion, Stack overflow

Usually caused by a bug in program logic, causing infinite recursive function calls.

How to fix this error: Check recursive functions for bugs that could cause them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to fix this error: Check that the call at the error’s line number gets correct input.

XMLHttpRequest cannot load http://some/url/. No ‘Access-Control-Allow-Origin’ header is present on the requested resource

Related errors: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://some/url/

This error is always caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is correct and it respects the same-origin policy. A good way to find the offending code is to look at the URL in the error message and find it from your code.

InvalidStateError: An attempt was made to use an object that is not, or is no longer, usable

Related errors: InvalidStateError, DOMException code 11

Means the code called a function that you should not call at the current state. Occurs usually with , when attempting to call functions on it before it’s ready.

var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val');

In this case, you would get the error because the function can only be called after calling .

How to fix this error: Look at the code on the line pointed by the error and make sure it runs at the correct time, or add any necessary calls before it (such as )


JavaScript has some of the most unhelpful errors I’ve seen, with the exception of the notorious in PHP. With more familiarity the errors start to make more sense. Modern browsers also help, as they no longer give the completely useless errors they used to.

What’s the most confusing error you’ve seen? Share the frustration in the comments!

Want to learn more about these errors and how to prevent them? Detect Problems in JavaScript Automatically with ESLint.

About Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies like Nokia and hot super secret startups. When not programming or playing games, Jani writes about JavaScript and high quality code on his site.


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