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Variables are containers for storing data (storing data values)
Variables are containers for storing data (storing data values).
In this example, x, y, and z,
are variables, declared with the var keyword:
var x = 5;
var y = 6;
var z = x + y;
In this example, x, y, and z,
are variables, declared with the let keyword:
let x = 5;
let y = 6;
let z = x + y;
In this example, x, y, and z,
are undeclared variables:
x = 5;
y = 6;
z = x + y;
From all the examples above, you can guess:
Always declare JavaScript variables with var,let,
orconst.
The var keyword is used in all JavaScript code from 1995 to 2015.
The let and const keywords were added to JavaScript in 2015.
If you want your code to run in older browser, you must use var.
If you want a general rule: always declare variables with const.
If you think the value of the variable can change, use let.
In this example, price1, price2, and total, are variables:
const price1 = 5;
const price2 = 6;
let total = price1 + price2;
The two variables price1 and price2
are declared with the const keyword.
These are constant values and cannot be changed.
The variable total is declared with the let keyword.
This is a value that can be changed.
Just like in algebra, variables hold values:
let x = 5;
let y = 6;
Just like in algebra, variables are used in expressions:
let z = x + y;
From the example above, you can guess that the total is calculated to be 11.
Variables are containers for storing values.
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:
JavaScript identifiers are case-sensitive.
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
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.
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.
const pi = 3.14;
let person = "John Doe";
let answer = 'Yes I am!';
Creating a variable in JavaScript is called "declaring" a variable.
You declare a JavaScript variable with the var or the let keyword:
var carName;
or:
let carName;
After the declaration, the variable has no value (technically it is undefined).
To assign a value to the variable, use the equal sign:
carName = "Volvo";
You can also assign a value to the variable when you declare it:
let carName = "Volvo";
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":
<p id="demo"></p>
<script>
let carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>
It's a good programming practice to declare all variables at the beginning of a script.
You can declare many variables in one statement.
Start the statement
with let and separate the variables by comma:
let person = "John Doe", carName = "Volvo", price = 200;
A declaration can span multiple lines:
let person = "John Doe",
carName = "Volvo",
price = 200;
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:
let carName;
If you re-declare a JavaScript variable declared with var, it will not lose its value.
The variable carName will still have the value "Volvo" after the execution of these statements:
var carName = "Volvo";
var carName;
You cannot re-declare a variable declared with let or const.
This will not work:
let carName = "Volvo";
let carName;
As with algebra, you can do arithmetic with JavaScript variables, using
operators like = and +:
let x = 5 + 2 + 3;
You can also add strings, but strings will be concatenated:
let x = "John" + " " + "Doe";
Also try this:
let x = "5" + 2 + 3;
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this:
let x = 2 + 3 + "5";
Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:
let $ = "Hello World";
let $$$ = 2;
let $myMoney = 5;
Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library.
In the JavaScript library jQuery, for instance, the main function
$ is used to select HTML elements.
In jQuery $("p"); means "select all p elements".
Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:
let _lastName = "Johnson";
let _x = 2;
let _100 = 5;
Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for "private (hidden)" variables.
Create a variable called carName and assign the value Volvo to it.
var = "";