JavaScript
JavaScript is a high-level, interpreted programming language for computers. It is often run in web browser applications to create dynamic content like a popup message or a live clock. It is not related to the programming language Java.
Although Java and JavaScript have many external similarities such as name and syntax, the two languages follow different design principles.
Contents
Example[change | change source]
Variables can be defined in several ways. In the older ES6 version, variable is defined using the var keyword[1]. In the newer versions after ES6, variable can be defined using const for constant variables and let for local variables [2][3]. The value of the constant variable cannot be re-declared or re-assigned. Local variables are declared inside the block scope.
1 //ES6
2 var x = 1; // defines the variable x and assigns the value 1 to the variable.
3
4 //ES6+
5 const y = 10; // defines the constant varialbe y and assigns the value 10 to the variable.
6 let t = 5; // defines the local varialbe t and assigns the value 10 to the variable.
The script below prints "Example" on the screen. The lines that start with// are comments; comments explain the purpose of the codes.[4]
1 <script type="text/javascript">
2 function example()
3 {
4 var ex = document.createTextNode('Example'); //make the computer remember "Example",
5 //so whenever you say "ex" the computer will append it with "Example"
6 document.body.appendChild(ex); //put the text on the bottom of the webpage
7 }
8 example();
9
10 /*
11 * The code below does almost the same thing as the code above,
12 * but it shows "Example" in a popup box and is shorter.
13 *
14 * This is a comment too, by the way.
15 */
16
17 alert("Example");
18 </script>
The JavaScript is enclosed by <script></script> html tags. The html tags indicate that it is a script and not text to be put onto the web page the JavaScript is running on. The script below inserts the numbers 1 through 10 at the bottom of a webpage:
1 <script type="text/javascript">
2 for(var numOfTimesAround = 1; numOfTimesAround <= 10; numOfTimesAround++){
3
4 document.body.innerHTML = document.body.innerHTML + numOfTimesAround + "<br>";
5 /*
6 * This puts the number, then a new line, at the end of the web page.
7 * In javascript, using the + sign combines two words together.
8 * So writing "Hello" + " World" would make "Hello World".
9 * Or, writing 1 + "<br>" makes "1<br>", which is what we want.
10 */
11
12 }
13
14 </script>
The for() loop makes whatever code is between the { and the } happen more than one time. In this case, it keeps looping until numOfTimesAround is equal to 10, then it stops.
Differences between Java and Javascript[change | change source]
- In Java, to define a variable, you have to say what type of variable it is: a number, a word, a letter, or more. In JavaScript, this is not necessary.
- In JavaScript, functions are stored as variables (unlike Java). This makes the following code okay in JavaScript:
function sayHi(){
alert("hi!");
}
sayHi = function(){
alert("Bye!");
}
sayHi();
//alerts "Bye!" instead of "hi!" without giving an error
- JavaScript runs in a web browser, but Java runs directly on a computer.
- JavaScript is interpreted, but Java, in most cases, must be compiled. This means that JavaScript needs to be run by another computer program (an interpreter), but finished Java can be run as its own program.
Related pages[change | change source]
References[change | change source]
Other websites[change | change source]
- Learn JavaScript on the Mozilla Developer Center
- Mozilla's Official Documentation on JavaScript
- Video - Firefox 2 and Javascript with Mozilla Corp and JavaScript creator Brendan Eich
- References for Core JavaScript versions: 1.5
- New in JavaScript: 1.7, 1.6
- List of JavaScript releases: versions 1.0 - 1.7
- Brendan's Roadmap Updates: JavaScript 1, 2, and in between - the author's blog entry
- comp.lang.javascript FAQ Official FAQ for the comp.lang.javascript Usenet group
- RFC 4329, a document for the registration of media types related to ECMAScript and JavaScript. The current recommendations are "application/javascript" and "application/ecmascript", although neither is recognized by Internet Explorer.
