JavaScript use (iii)

1.JavaScript Variables

A variable is a "container" for storing information.

var x=5; var y=6; var z=x+y;

1.1 is like algebra.

X=5
Y=6
Z=x+y

In algebra, we use letters (such as x) to hold values (such as 5).

By the above expression Z=x+y, we can calculate the value of Z as 11.

In JavaScript, these letters are called variables.

1.2JavaScript variables

Like algebra, JavaScript variables can be used to hold values (such as x=5) and expressions (such as z=x+y).

Variables can use short names (such as x and y), or they can use better descriptive names (such as age, Sum, totalvolume).

• Variables must start with a letter
• Variables can also start with the $ and _ symbols (although we do not recommend this)
• Variable names are case sensitive (Y and y are different variables)
• Both JavaScript statements and JavaScript variables are case-sensitive.

1.3JavaScript Data types

JavaScript variables can also hold other data types, such as text values (name= "Bill Gates").

In JavaScript, a text like "Bill Gates" is called a string.

There are many types of JavaScript variables, but for now, we only focus on numbers and strings.

When you assign a text value to a variable, enclose the value in double or single quotation marks.

Do not use quotation marks when the value you assign to a variable is numeric. If you enclose a value in quotation marks, the value is treated as text.

var pi=3.14; var person="John Doe"; var answer='Yes I am! ';

1.4 declaring (creating) JavaScript variables

Creating a variable in JavaScript is often referred to as a "declaration" variable.

We use the VAR keyword to declare variables:

var carname;

After the variable declaration, the variable is empty (it has no value).

To assign a value to a variable, use the equals sign:

Carname="Volvo";

However, you can also assign a value to a variable when you declare it:

var carname="Volvo";

In the following example, we create a variable named Carname and assign it a value of "Volvo" and put it in an HTML paragraph id= "Demo":

<p id="demo"></p>var carname="Volvo"  ;d Ocument.getelementbyid ("demo"). Innerhtml=carname;

A good programming habit is to declare the required variables uniformly at the beginning of the code.

1.5 One statement, multiple variables

You can declare many variables in a single statement. The statement starts with VAR and separates the variables with commas:

var lastname="Doe", age=, job="Carpenter" ;

Declarations can also span multiple lines:

var lastname="Doe",age =$, job="  Carpenter";

1.6Value = undefined

In computer programs, variables that are not valued are often declared. A variable that is not declared with a value, whose value is actually undefined.

After the following statement has been executed, the value of the variable carname will be undefined:

var carname;

1.7 re-declaring JavaScript variables

If you re-declare a JavaScript variable, the value of the variable is not lost:

After the following two statements are executed, the value of the variable carname remains "Volvo":

var carname="Volvo"var carname;

1.8JavaScript arithmetic

You can use JavaScript variables to do arithmetic, using the operators such as = and +:

y=5; x=y+2;

JavaScript use (iii)