Let’s now see an overview of the main Javascript objects to use to start taking the first steps in programming with this language.
Window And DOM Object
The essential item for cooperating with the program is a window containing an HTML record. Every window or tab has its related window object, and each edge characterized in an HTML page compares to a window object. This article, as well as distinguishing the visual component of the program, addresses the worldwide execution setting for JavaScript, that is, the item inside which worldwide factors and works are characterized. Any factor or capability characterized in the worldwide setting turns into a property or technique for the window object.
Among the techniques: alert () shows a modular window with a message and an OK button; affirm () indicates a modular window with a message and two buttons (one for affirmation and one for drop) and returns a Boolean worth addressing valid or misleading; brief () requires the contribution of a price that is caught and produced by the strategy; open () opens another window or tab. The window object additionally gives a few instruments to oversee and control client route: history, which monitors the way, and area, which permits you to deal with the persistent URL.
To acquire data on the ongoing program and a portion of its settings, we can utilize the guide property, which gives data to recognize the program and the working framework on which it is running. Yet, the data isn’t generally so valuable and precise. The appName property, which ought to return the program name, returns items that are unimportant all the time. One more of the window object properties is the archive which addresses the HTML report stacked in the ongoing window and the design of this article, known as the DOM. To see the tree construction of a record, you can utilize one of the examination devices coordinated or generally accessible in the latest programs.
The getElementById () method returns an object representing the element type node with the id attribute with the specified value. The getElementsByName () method is analogous, producing the list of page nodes whose name attribute value corresponds to that of the parameter. Unlike the first, this method returns a list of objects, a NodeList, and a data structure similar to an array containing DOM nodes. It is possible to locate the elements of a page based on their tag using the getElementsByTagName () method, which returns in the form of a NodeList the list of nodes corresponding to the label specified as a parameter.
With the getElementByClassName () method, you get the list of nodes to which a specific value has been assigned as a class attribute. Among the new features introduced most recently in the DOM, specifications are the possibility to select the elements of a page using the CSS selectors: querySelector (), which returns the first element found, and querySelectorAll (), the list of all the features found from the selector. Once the element or elements on a page have been identified, we can modify its content by exploiting specific properties and methods of element type nodes. Some DOM methods allow us to analyze and move within the structure of a document.
Event Object
In addition to being organized according to objects and methods, JavaScript exploits the presence of events: it is something that happens in the document in part due to the occurrence of specific user interaction (hovering over a link, clicking on something …), other times to the stresses coming from other applications or from the system itself (the page has been loaded). To intercept the triggered events, we use the handler (or listener) mechanism: a callback function associated with a particular event.
The addEventListener () function is a method exposed by the elements of the DOM and represents the most common of the ways used to associate an event to the respective handler, it allows you to define and manage more than one for the same event, and it works with any element of the DOM (not just with HTML elements). Another advantage: we can remove the association between event and handler using the removeEventListener function, which takes the same parameters as addEventListener but has the opposite function.
The target property represents the element on which the event occurred, independently of other factors such as the flow of events, and therefore with more excellent guarantees than this object. Together with the event’s element, we can also identify the type of event through the type property.
Access A Form
A form is a component of a Web page that, through particular elements called controls, allows the user to enter data to be sent to the server for processing. Controls are the elements that will enable the user to enter data, select values , or start functions, such as text boxes, checkboxes, radio buttons, buttons, and the like. For simple forms, collecting and sending data to the server can be carried out without the need to involve JavaScript, especially with the introduction of the new HTML5 features that provide for initial validation of the data entered by the user.
However, to have some control over the data, especially if the processing is to take place on the client, it is necessary to resort to JavaScript. During the generation of the DOM, an array containing the list of objects representing the forms present on the page is populated. This array is accessible through the forms property of the document object. The best approach is to give the state an identifier via the id attribute or a name via the name attribute. Another way to access a named form is to use the getElementsByName () method, which allows you to access any element that has been called via the name attribute.
Note that while the id attribute is used to identify an element in the DOM, and therefore there should be no multiple elements with the same id, it is possible to have multiple elements with the same value for the name attribute. For this reason the getElementsByName () method returns an array of objects. Each form object contains an array containing its controls that can be indexed using the form object. Also, for the rules, it is possible to exploit the name attribute and access them, as we have seen for the forms.
Image Object
JavaScript can manipulate images at varying levels of complexity within an HTML page. The recent technologies linked to the developments of HTML5 allow for the management of ready-made photos and dynamically generate and modify graphics. The most basic approach to managing an image is to manipulate the HTML “img” element: a static image on the page that needs no processing by JavaScript.
By controlling the “img” element via JavaScript, we can create simple functions such as browsing through a list of images. You can create an image element to add to the DOM using the Image () constructor: a similar approach to creating an “img” element but convenient for preloading images. Each Image is loaded when we assign the file name to the src attribute. This can lead to a wait on the user’s part, especially if the Image size is not negligible. Creating an Image object causes the corresponding Image to be loaded into the browser cache without being displayed on the page.
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