Introduction to JavaScript

JS is very much like Java in syntax, although execution engine is very different.

Following constructs are exactly same:

1. Comments: both // and /**/

2. Loops: for, while, do-while + an extra for (<var> in <values)

3. Statements: if, else, switch

4. try catch, throw

5. Operators: ++, --, += etc.

6. Strings behave like Java objects and have all Java String functions available like length(), toUpperCase() etc.

JS does everything by referring to a variable called document which refers to the current page.

Example:

1. document.write (“Hello World”);

2. document also has a map called forms which enables to get any form in the page by its name. Example:

<form name="myForm" action="demo_form.asp" onsubmit="return validateForm()" method="post">

    First name: <input type="text" name="fname">
    <input type="submit" value="Submit">

</form>


<script>
function validateForm() {

    var x=document.forms["myForm"]["fname"].value;
    // do some validation here
    return true;
}
</script>

JS can basically access any field of an HTML construct as shown above.

So, this can also be used to add dynamism to web pages by calling JS scripts on events.