Variables are Containers for Storing Data
In this first example,
are undeclared variables.
They are automatically declared when first used:
y = 6;
z = x + y;
It is considered good programming practice to always declare variables before use.
From the examples you can guess:
- x stores the value 5
- y stores the value 6
- z stores the value 11
Example using var
var y = 6;
var z = x + y;
var keyword should only be used in code written for older browsers.
Example using let
let y = 6;
let z = x + y;
Example using const
const y = 6;
const z = x + y;
const price2 = 6;
let total = price1 + price2;
The two variables
are declared with the
These are constant values and cannot be changed.
total is declared with the
total can be changed.
When to Use var, let, or const?
1. Always declare variables
2. Always use
const if the value should not be changed
3. Always use
const if the type should not be changed (Arrays and Objects)
4. Only use
let if you can't use
5. Only use
var if you MUST support old browsers.
Just Like Algebra
Just like in algebra, variables hold values:
let y = 6;
Just like in algebra, variables are used in expressions:
From the example above, you can guess that the total is calculated to be 11.
Variables are containers for storing values.
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
=) is an "assignment" operator, not an
"equal to" 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.)
The "equal to" operator is written like
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.
let person = "John Doe";
let answer = 'Yes I am!';
var or the
After the declaration, the variable has no value (technically it is
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":
let 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
let 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
The variable carName will have the value
undefined after the execution of this statement:
var, it will not lose its value.
carName will still have the value "Volvo" after the execution of these statements:
You cannot re-declare a variable declared with
This will not work:
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:
let $$$ = 2;
let $myMoney = 5;
$ is used to select HTML elements.
$("p"); means "select all p elements".
let _x = 2;
let _100 = 5;