Java Script Lesson

Java Script Lesson Java Script - Lesson 2 Instructor: Roseann Krane

A JavaScript script that displays a default message in the browser status bar when a user loads a web page may be created.

Event handlers: JavaScript is an event-driven language.

An event is a user or system action:

a key press
a mouse click
the selection of text
the loading of a Web page
the passage of 500 milliseconds, and so on.

JavaScript responds to an event by running its associated event handler code. The syntax of an event handler is:

onEvent="code"

Events can be one of several events, including:
- Click--clicking the mouse on a button, check box, link, etc.
- Load--loading a Web page
- MouseOver--pointing the mouse to a link

The onLoad event handler is called when the browser finishes loading a page. This gives the user a chance to personalize a page at startup. The syntax of the onLoad event handler:

onLoad="code"

The onLoad event handler must be inserted in the HTML document's BODY tag. Or insert it in the FRAMESET tag of the page's parent frame document when working with multi-frame pages.

JavaScript is an object-based scripting language.

An object is a programming construct with properties and methods.
A property is a variable associated with an object
A method is a function associated with an object.
Here is the syntax used to refer to a property:

object.property

Syntax used to call a method:

object.method([param1, param2, ..])

JavaScript provides a generous selection of built-in objects -- window, document, form, link, anchor, text, button, check box, and more -- along with their associated properties and methods. You can also create your own objects.

The window object is the top-level built-in JavaScript object. All other JavaScript objects are contained within the window object. It is a two-part expression:

window.defaultStatus

window is the top-level built-in JavaScript object and contains all other JavaScript objects.

defaultStatus is a property of the window object. The defaultStatus property specifies a default message in the status bar at the bottom of the browser window.

window.defaultStatus uses standard dot notation (object.property) to denote the defaultStatus property of the window object.

JavaScript scripts can be embedded or nonembedded. An embedded script is one that is located within an HTML tag. Example:

A nonembedded script is one that is located between <SCRIPT LANGUAGE="JavaScript"> .. tags. Example:

<SCRIPT LANGUAGE="JavaScript">
    var currDate = new Date()
    var currDay = currDate.getDate()
    var currMonth = currDate.getMonth() + 1
    var currYear = currDate.getYear() currDateStr = currMonth + "/" + currDay + "/" + currYear
</SCRIPT>

Assignment -- Add the following to the body line of your web page:

onLoad="window.defaultStatus='Welcome to my Web page!'"

Questions: (E-mail the answers to your teacher.)

What is the instruction called when a statement appears in the browser status bar? (onEvent or onLoad) Explain.
Name three of the actions that can happen by an event call.
Explain embedded script.

END of Lesson.