Variables
A variable is a named storage for data to access it later in the program. There are three ways to create variables.
| Keyword | Description | Scope |
|---|---|---|
| var | Var is used to declare variables(old way of declaring variables) | Function or global scope |
| let | let is also used to declare variables(new way) | Global or block Scope |
| const | const is used to declare const values. Once the value is assigned it can not be modified | Global or block Scope |
Example program to understand the difference between var, let and const
Check the below program to understand issues with var. In the below example, age is a var variable whose value is not getting reflected back to 20 even after coming out of if block but with let, name will again becomes foo. Hence, let is recommended to use.
let name ="foo";
console.log('name:', name); //global scope, here name = foo
const id =1;
console.log('id:', id); //global scope, here id=1
var age = 20;
console.log('age:', age); //global scope, here age = 20
if(id) {
let name ="bar"; //block scope
console.log('inside block name:', name); //here name = bar
//const id =1;
// if you uncomment the above line, program will throw an error as const can't be assigned twice
console.log('id:', id); //id =1
var age = 25;
console.log('age:', age); // here age=25
}
console.log('outside block name:', name); //here name will be foo again
console.log('id:', id); //id =1
console.log('age:', age); // age will not become 20 and it remains 25 even after the block is ended
Try yourself here
How to declare variables
let variable-name; // Just declaration
let variable-name = value; // declaring variable and assigning it with some value
let var1 = value1, var2 = value2, var3 = value3; // multiple variables declaration with their assigned values
You can change the value of a variable multiple times with different data, Javascript will not throw any error if you assign string value to a integer variable.
let x = 10;
x= "hello"; // don't throw any error and x will assigned to "hello"