In this example, x, y, and z, are variables:
var y = 6;
var z = x + y;
From the example above, you can expect:
- x stores the value 5
- y stores the value 6
- z stores the value 11
Much Like Algebra
In this example, price1, price2, and total, are variables:
var price2 = 6;
var total = price1 + price2;
In programming, just like in algebra, we use variables (like price1) to hold values.
In programming, just like in algebra, we use variables in expressions (total = price1 + price2).
From the example above, you can calculate the total to be 11.
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:
- 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)
The Assignment Operator
This is different from algebra. The following does not make sense in algebra:
(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)
In programming, text values are called text 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.
var person = "John Doe";
var answer = 'Yes I am!';
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":
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
It's a good programming practice to declare all variables at the beginning of a script.
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:
carName = "Volvo",
price = 200;
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:
The variable carName will still have the value "Volvo" after the execution of these statements:
You can also add strings, but strings will be concatenated:
Also try this:
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this: