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When you want to use JavaScript with a web page, you use the HTML

<script> element to tell the browser it is coming across a script. Its s re attribute tells people where the JavaScript file is stored. ## You may see JavaScript in the HTML between opening <script> and closing </script> tags

(but it is better to put scripts in their own files).

When the browser comes across a <script> element, it stops to

load the script and then checks to see if it needs to do anything.

It is best to keep JavaScript code in its own JavaScript

file. JavaScript files are text files (like HTML pages and CSS style sheets), but they have the . j s extension. The HTML <script> element is used in HTML pages to tell the browser to load the JavaScript file (rather like the element can be used to load a CSS file). If you view the source code of the page in the browser, the JavaScript will not have changed the HTML, because the script works with the model of the web page that the browser has created.

A script is a series of instructions that a computer can follow one-by-one.

Each individual instruction or step is known as a statement. Statements should end with a semicolon.

You should write comments to explain what your code does.

They help make your code easier to read and understand. This can help you and others who read your code.

A script will have to temporarily

store the bits of information it needs to do its job. It can store this data in variables.

A variable is a good name for this

concept because the data stored in a variable can change (or vary) each time a script runs.

JavaScript distinguishes between numbers,

strings, and true or false values known as Booleans.

  • NUMERIC DATA TYPE

  • STRING DATA TYPE

  • BOOLEAN DATA TYPE

You can also use a technique

called escaping the quotation characters. This is done by using a backwards slash (or "backslash") before any type of quote mark that appears within a string (as shown on the fourth line of this code sample). The backwards slash tells the interpreter that the following character is part of the string, rather than the end of it.

A Boolean variable can only have

a value of true or fa 1 se, but this data type is very helpful.

Programmers sometimes use

shorthand to create variables. Here are three variations of how to declare variables and assign them values

Once you have assigned a value

to a variable, you can then change what is stored in the variable later in the same script.

Here are six rules you must always follow when giving a variable a name:

  • The name must begin with a letter, dollar sign ($),or an underscore (_). It must not start with a number

  • The name can contain letters, numbers, dollar sign ($), or an underscore (_). Note that you must not use a dash(-) or a period (.) in a variable name.

  • You cannot use keywords or reserved words. Keywords are special words that tell the interpreter to do something. For example, var is a keyword used to declare a variable. Reserved words are ones that may be used in a future version of JavaScript. ONLINE EXTRA View a full list of keywords and reserved words in JavaScript

  • All variables are case sensitive, so score and Score would be different variable names, but it is bad practice to create two variables that have the same name using different cases.

  • Use a name that describes the kind of information that the variable stores. For example, fi rstName might be used to store a person's first name, l astNarne for their last name, and age for their age.

  • If your variable name is made up of more than one word, use a capital letter for the first letter of every word after the first word. For example, f i rstName rather than fi rstnarne (this is referred to as camel case). You can also use an underscore between each word (you cannot use a dash).