Can you explain the meaning of `this.$something = this.$('something')`?

I'm currently diving into backbone fundamentals and feeling a bit lost with the following line of code:

this.$input = this.$('#new-todo');

Can someone explain to me what exactly is happening here?

As far as I can tell, this.$('#new-todo') seems like a standard jQuery selector that targets the #new-todo (which is an input field) and assigns it to this.$input. According to this resource, $(this + 'input') is just a shorthand for this, but this is where I am confused - why is that necessary? Is it simply moving the #new-todo element from the DOM to $(this + 'input')? If so, wouldn't it be more straightforward to use this.$('something') instead of this.$input?

javascriptbackbone.js

Answer №1

The snippet of code showcases the concept of caching a jQuery element.

When utilizing the jQuery constructor, it involves querying the Document Object Model (DOM) to locate all matching elements. This process can be resource-intensive depending on the selector used.

If there are intentions to reuse a jQuery object multiple times, it is more efficient to query the DOM just once.

Consider this demonstration:

var foo = $('.someClass');
foo.css("background-color", "yellow");
console.log(foo.length + " items updated");

Despite referencing foo several times in the example above, the DOM query occurred only once.

Wouldn't it be beneficial if the fact that foo represents a jQuery object was more evident in your code?

// Many developers opt to prefix variable names with $
var $foo = $('.someClass');

Answer №2

Backbone.$ points to either jQuery or Zepto.

When using this.$, it will direct to jQuery and retrieve an element identified by the id new-todo

this.$input follows a convention indicating that $input is a jQuery object.

The reference to this object pertains to a Backbone instance, not a DOM context.

this.$ == jQuery

this.$input solely serves as an attribute.

The line 

this.$input = this.$("#new-todo");
could also be stated as:

this.input = jQuery("#new-todo");
or

this.input = $("#new-todo");

$input acts as a random property name, similar to how variables can have names like $foo, $bar, etc.

Answer №3

Using $(this + 'input') does not seem to be a valid syntax, as it would require this to be a string. I am not familiar with Backbone, but it appears that this.$ represents jQuery in this context. Therefore, this.$("#new-todo") in Backbone is equivalent to jQuery('#new-todo'), which simply selects the specified element. By storing this selection in a variable named $input, you can reduce the number of times the selector function is called and easily reference this.$input in other methods without needing to remember the original selector.

Answer №4

this.$input is essentially just a property of the object that this points to.

Using this.$('#new-todo') will search for and return the corresponding element within the current view (the element specified by el in the view).

When we employ this.$, as per the explanation in the documentation, the searches to locate the element indicated by the selector are contained within the view's element (referred to as el). You can learn more about what "scope" means in jQuery by checking out the definition of context. Here is where you can find details on this concept.

Therefore, using this.$('#new-todo') doesn't produce the same result as using jQuery('#new-todo').

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