Modern HTML5 Techniques for changing classes
Modern browsers have added classList which provides methods to make it easier to manipulate classes without needing a library:
document.getElementById("MyElement").classList.add('MyClass');
document.getElementById("MyElement").classList.remove('MyClass');
if ( document.getElementById("MyElement").classList.contains('MyClass') )
document.getElementById("MyElement").classList.toggle('MyClass');
Unfortunately, these do not work in Internet Explorer prior to v10, though there is a shim to add support for it to IE8 and IE9, available from this page. It is, though, getting more and more supported.
Simple cross-browser solution
The standard JavaScript way to select an element is using document.getElementById("Id")
, which is what the following examples use - you can of course obtain elements in other ways, and in the right situation may simply use this
instead - however, going into detail on this is beyond the scope of the answer.
To change all classes for an element:
To replace all existing classes with one or more new classes, set the className attribute:
document.getElementById("MyElement").className = "MyClass";
(You can use a space-delimited list to apply multiple classes.)
To add an additional class to an element:
To add a class to an element, without removing/affecting existing values, append a space and the new classname, like so:
document.getElementById("MyElement").className += " MyClass";
To remove a class from an element:
To remove a single class to an element, without affecting other potential classes, a simple regex replace is required:
document.getElementById("MyElement").className =
document.getElementById("MyElement").className.replace
( /(?:^|\s)MyClass(?!\S)/g , '' )
/* Code wrapped for readability - above is all one statement */
An explanation of this regex is as follows:
(?:^|\s) # Match the start of the string or any single whitespace character
MyClass # The literal text for the classname to remove
(?!\S) # Negative lookahead to verify the above is the whole classname
# Ensures there is no non-space character following
# (i.e. must be the end of the string or space)
The g
flag tells the replace to repeat as required, in case the class name has been added multiple times.
To check if a class is already applied to an element:
The same regex used above for removing a class can also be used as a check as to whether a particular class exists:
if ( document.getElementById("MyElement").className.match(/(?:^|\s)MyClass(?!\S)/) )
### Assigning these actions to onclick events:
Whilst it is possible to write JavaScript directly inside the HTML event attributes (such as onclick="this.className+=' MyClass'"
) this is not recommended behaviour. Especially on larger applications, more maintainable code is achieved by separating HTML markup from JavaScript interaction logic.
The first step to achieving this is by creating a function, and calling the function in the onclick attribute, for example:
<script type="text/javascript">
function changeClass(){
// Code examples from above
}
</script>
...
<button onclick="changeClass()">My Button</button>
(It is not required to have this code in script tags, this is simply for the brevity of example, and including the JavaScript in a distinct file may be more appropriate.)
The second step is to move the onclick event out of the HTML and into JavaScript, for example using addEventListener
<script type="text/javascript">
function changeClass(){
// Code examples from above
}
window.onload = function(){
document.getElementById("MyElement").addEventListener( 'click', changeClass);
}
</script>
...
<button id="MyElement">My Button</button>
(Note that the window.onload part is required so that the contents of that function are executed after the HTML has finished loading - without this, the MyElement might not exist when the JavaScript code is called, so that line would fail.)
JavaScript Frameworks and Libraries
The above code is all in standard JavaScript, however, it is common practice to use either a framework or a library to simplify common tasks, as well as benefit from fixed bugs and edge cases that you might not think of when writing your code.
Whilst some people consider it overkill to add a ~50 KB framework for simply changing a class, if you are doing any substantial amount of JavaScript work or anything that might have unusual cross-browser behavior, it is well worth considering.
(Very roughly, a library is a set of tools designed for a specific task, whilst a framework generally contains multiple libraries and performs a complete set of duties.)
The examples above have been reproduced below using jQuery, probably the most commonly used JavaScript library (though there are others worth investigating too).
(Note that $
here is the jQuery object.)
Changing Classes with jQuery:
$('#MyElement').addClass('MyClass');
$('#MyElement').removeClass('MyClass');
if ( $('#MyElement').hasClass('MyClass') )
In addition, jQuery provides a shortcut for adding a class if it doesn't apply, or removing a class that does:
$('#MyElement').toggleClass('MyClass');
### Assigning a function to a click event with jQuery:
$('#MyElement').click(changeClass);
or, without needing an id:
$(':button:contains(My Button)').click(changeClass);
Best Answer
It's important to know that this syntax:
is just syntactic sugar for creating an instance method in the class constructor:
Note: This syntax is not an official part of the JavaScript language yet (currently in stage 3) so you must use a transpiler like Babel to handle it.
The value of
this
withinmethod
is the classA
because that is whatthis
points to in the constructor (since arrow functions inherit the context from the scope they are defined in):Defining a regular (non-arrow function) method on the class creates a method on the class prototype (not instance) but sets no rules on what
this
will be (sincethis
is dynamic in JS and depends on how a function is called, not how it's defined).If methods defined in either of these ways are called on the class instance (via the
.
), as per the rule of howthis
is bound when a function is called as a method of an object,this
will point to the class instance in both cases:One major difference between the two method declarations above is that the instance method has
this
always fixed to the class instance while the class (prototype) method does not (we can change it by using Function.prototype.apply or Function.prototype.call)A common occurrence where the
this
changes is within an event handler, where the event handler calls the function passed into it and binds the context to the element on which the event happened (so overrides the value ofthis
to be the element that was clicked or whatever the event was)This happens in React as well for all (synthetic) DOM event handlers.
Therefore, if we want our method's context to always point to the instance of the React component, we can use the instance method.
Another way of restricting the context but not using the special instance method syntax that requires Babel is to directly create an instance method ourselves by creating a new function from the class (prototype) method with a bound context (using Function.prototype.bind):
This allows us to arrive to the same result as using the special instance method syntax but with the currently available tools (ES2017 and under).
If for some reason we want a method that is always bound to something that is not an instance of the class, we can do that as well: