JavaScript Variables

Charles Russell

Bennu Bird Media

Statement and Expressions

● A Statement requests an action.● An Expression sets a value.● Variables are used in both

● They are passed into functions and used to control branching and iteration

● They can be the target of the expression or the value of the expression– In computer books you may see L value and R Value when

referring to expressions

So What are they

● A variable a just a place to put things you want to use later. A good anology would be a box

● In JavaScript anything can go into a box● In this language the boxes are a little magical

– Boxes can resize themselves to hold anything you put in them.

– Dumping from one box into another makes what was already the box you are dumping into vanish

Labeling the Boxes

● To name a variable just type a name and set a value. You will not only named it you have created and sized it.● var myVariable=“Some really cool stuff“;

– Note the var while not required leaving it off has some implications that we will get into later

● Another word for name is identifier and in computer books it is more common to use identifier.

● In general you can use any name you want, but there are a few rules

Whats in a name

● In Javascript a name must begin with a letter _ or $● Names beginning with _ are generally used by

the system● Names beginning with $ are commonly used by

libraries● After the first character you can use as many

letters and numbers $ and _ as you want after that

● Name are case sensitive Name and name are not the same thing to the interpreter

Forbiden Names

● Use of these words as names will cause an errorabstract debugger final instanceof protected throws

boolean default finally int public transient

break delete float interface return true

byte do for let short try

case double function long static typeof

catch else goto native super var

char enum if new switch void

class export implements null synchronized volatile

const extends import package this while

continue false in private throw with

Names with Baggage

● Some names are used by Javascript implementations and while legal should be avoided. A partial list is below for more go to http://www.javascripter.net/faq/reserved.htm

alert clearInterval decodeURIComponent

all clearTimeout defaultStatus

anchor clientInformation document

anchors close element

area closed elements

Array confirm embed

assign constructor embeds

blur crypto encodeURI

button Date encodeURIComponent

checkbox decodeURI escape

Weak Typing

● JavaScript is a weakly typed language● This does not mean that it is type unaware

● This means that you do not have to declare the type of the variable

Summary

● Variables are used in statements and expressions● The are just a way to store things for later use● There are rules to names● You cant use reserved words as names● Some legal names are not a good Idea● Java Script is a weekly typed language

Next Datatypes