JavaScript Variables and Constants
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JavaScript Variables
In programming, a variable is a container (storage area) to hold data. For example,
let num = 5;
Here, num is a variable. It’s storing 5.
JavaScript Declare Variables
In JavaScript, we use either var or let keyword to declare variables. 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 later tutorials). |
let is block scoped (will be discussed in later tutorials). |
For example, var x; |
For example, let y; |
Note: It is recommended we use let instead of var. However, there are a few browsers that do not support let. Visit JavaScript let browser support to learn more.
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
// vaue of variable x is changed
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 initializing 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. Visit JavaScript const browser support to learn more.
Now that you know about variables, you will learn about different types of data a variable can store in the next tutorial.
