(JavaScript Tutorials for Beginners)
In this end-to-end example, you will learn – JavaScript tutorials for Beginners – JavaScript Variables and Constants.
JavaScript Variables and Constants
In this tutorial, you will learn about JavaScript variables and constants, and also how to initialize and use them with the help of examples.
JavaScript Variables
In programming, a variable is a container (storage area) to hold data.
JavaScript Declare Variables
In JavaScript, we use the following keywords to declare variables: var
and let
. For example,
var x;
let y;
Here, x and y are variables.
JavaScript var Vs let
Both var
and let
are used to declare variables. However, there are some differences between them.
var | let |
---|---|
var is used in the older versions of JavaScript |
let is the new way of declaring variables starting ES6 (ES2015). |
var is function scoped (will be discussed in the later tutorials). |
let is block scoped (will be discussed in the later tutorials). |
For example, var x; |
For example, let y; |
Note: It is recommended to use let
instead of var
. However, there are a few browsers that do not support let
.
JavaScript Initialize Variables
We use the assignment operator =
to assign a value to a variable.
let x;
x = 5;
Here, 5 is assigned to variable x.
You can also initialize variables during its declaration.
let x = 5;
let y = 6;
In JavaScript, it’s possible to declare variables in a single statement.
let x = 5, y = 6, z = 7;
If you use a variable without initializing it, it will have an undefined
value.
let x; // x is the name of the variable
console.log(x); // undefined
Here x is the variable name and since it does not contain any value, it will be undefined.
You will learn about undefined
and other data types in the next tutorial in detail.
Change the Value of Variables
It’s possible to change the value stored in the variable. For example,
// 5 is assigned to variable x
let x = 5;
console.log(x); // 5
// variable x is changed 3
x = 3;
console.log(x); // 3
The value of a variable may vary. Hence, the name variable.
Rules for Naming JavaScript Variables
The rules for naming variables are:
- Variable names must start with either a letter, an underscore
_
, or the dollar sign$
. For example,//valid let a = 'hello'; let _a = 'hello; let $a = 'hello';
- Variable names cannot start with numbers. For example,
//invalid Let 1a = 'hello'; // this gives an error
- JavaScript is case-sensitive. So y and Y are different variables. For example,
let y = "hi"; let Y = 5; console.log(y); // hi console.log(Y); // 5
- Keywords cannot be used as variable names. For example,
//invalid let new = 5; // Error! new is a keyword.
Notes:
- Though you can name variables in any way you want, it’s a good practice to give a descriptive variable name. If you are using a variable to store the number of apples, it better to use apples or numberOfApples rather than x or n.
- In JavaScript, the variable names are generally written in camelCase if it has multiple words. For example, firstName, annualSalary, etc.
JavaScript Constants
The const
keyword was also introduced in the ES6(ES2015) version to create constants. For example,
const x = 5;
Once a constant is initialized, we cannot change its value.
const x = 5;
x = 10; // Error! constant cannot be changed.
console.log(x)
Simply, a constant is a type of variable whose value cannot be changed.
Also, you cannot declare a constant without initiating it. For example,
const x; // Error! Missing initializer in const declaration.
x = 5;
console.log(x)
Note: If you are sure that the value of a variable won’t change throughout the program, it’s recommended to use const
. However, there are a few browsers that do not support const
.
Now that you know about variables, you will learn about different types of data a variable can store in the next tutorial.
Disclaimer: The information and code presented within this recipe/tutorial is only for educational and coaching purposes for beginners and developers. Anyone can practice and apply the recipe/tutorial presented here, but the reader is taking full responsibility for his/her actions. The author (content curator) of this recipe (code / program) has made every effort to ensure the accuracy of the information was correct at time of publication. The author (content curator) does not assume and hereby disclaims any liability to any party for any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result from accident, negligence, or any other cause. The information presented here could also be found in public knowledge domains.
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