Javascript Variables

Question 1

JavaScript variables are containers for storing data values.

In this example, x, y, and z, are variables:

From the example above, you can expect:

x stores the value 5
y stores the value 6
z stores the value 11

Answer 1

script>
var x = 5;
var y = 6;
var z = x + y;
document.getElementById("demo").innerHTML =
"The value of z is: " + z;
/script>

JavaScript variables are containers for storing data values.

Question 2

JavaScript Identifiers
All JavaScript variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

Answer 2

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with a letter
Names can also begin with $ and _ (but we will not use it in this tutorial)
Names are case sensitive (y and Y are different variables)
Reserved words (like JavaScript keywords) cannot be used as names
JavaScript identifiers are case-sensitive.

Question 3

The Assignment Operator
In JavaScript, the equal sign (=) is an “assignment” operator, not an “equal to” operator.

This is different from algebra. The following does not make sense in algebra:

x = x + 5

Answer 3

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.

(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)

The “equal to” operator is written like == in JavaScript.

Question 4

JavaScript Data Types
JavaScript variables can hold numbers like 100 and text values like “John Doe”.

In programming, text values are called text strings.

JavaScript can handle many types of data, but for now, just think of numbers and strings.

Strings are written inside double or single quotes. Numbers are written without quotes.

If you put a number in quotes, it will be treated as a text string.

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

Answer 4

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

document.getElementById(“demo”).innerHTML =
pi + “<br></br>” + person + “<br></br>” + answer;
/script>

Question 5

Declaring (Creating) JavaScript Variables
Creating a variable in JavaScript is called “declaring” a variable.

You declare a JavaScript variable with the var keyword:

After the declaration, the variable has no value (technically it has the value of undefined).

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

You can also assign a value to the variable when you declare it:

In the example below, we create a variable called carName and assign the value “Volvo” to it.

Then we “output” the value inside an HTML paragraph with id=”demo”:

It’s a good programming practice to declare all variables at the beginning of a script.

Answer 5

script>
var carName = “Volvo”;
document.getElementById(“demo”).innerHTML = carName;
/script>

Question 6

One Statement, Many Variables
You can declare many variables in one statement.

Start the statement with var and separate the variables by comma:

A declaration can span multiple lines:

Answer 6

var person = “John Doe”, carName = “Volvo”, price = 200;

var person = "John Doe",
carName = "Volvo",
price = 200;

Question 7

Value = undefined
In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.

A variable declared without a value will have the value undefined.

The variable carName will have the value undefined after the execution of this statement:

Answer 7

script>
var carName;
document.getElementById(“demo”).innerHTML = carName;

/script>

Question 8

JavaScript Arithmetic
As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

You can also add strings, but strings will be concatenated (side by side):

If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated. ( x = “5” + 2 + 3; translates to )

Now this (var x = 2 + 3 + “5” translates to)

Answer 8

var x = 5 + 2 + 3;

var x = “John” + “ “ + “Doe”;

523

55