var:

• The oldest way to declare variables.
• The behavior is either function-scoped or not block-scoped, so it can be globally-scoped as well.
• It can be redeclared and updated.

                                  
                                    var message = "Hello, World!";
var count = 10;

// It can be updated
count = count + 5;

// It can be redeclared
var message = "New message";                             
                                  
                                

let:

• Introduced in ECMAScript 6 (ES6).
• It has block-scoped behavior.
• It is not possible to either replace or redeclare the same in the same scope.

                                  
                                    let name = "John";
let age = 25;

// It can be updated
age = age + 1;

// It cannot be redeclared in the same scope
// let name = "Jane"; // This would throw an error              
                                  
                                

const:

• Also introduced in ECMAScript 6 (ES6).
• It has block-scoped behavior.
• It cannot be updated or redeclared.

                                  
                                    const pi = 3.14;
const language = "JavaScript";

// Cannot be updated
// pi = 3.14159; // This would throw an error

// Cannot be redeclared in the same scope
// const pi = 3.14159; // This would throw an error
                                  
                                

As a rule, you should always adopt a let over a const, if the latter does not require rare reassignments.

In order to use the function again and prevent the accidental reassignment of the variable, the function automatically provides this feature.