When we construct new literal expressions with const assertions, we can signal to the language that:

• no literal types in that expression should be widened (e.g. no going from "hello" to string)
• object literals get readonly properties
• array literals become readonly tuples

and here is a const assertion in action:

const email = 'john@gmail.com'; // type 'john@gmail.com'
const phones = [8494922901, 1238399293]; // type number[]
const session = { id: 123, name: 'x1keck323jKJdf1' };
// type 
// {
//     id: number;
//     name: string;
// }

const username = 'John' as const; // type 'John'
const roles = [ 'read', 'write'] as const; // type readonly ["read", "write"]
const address = { street: 'Tokyo', country: 'Japan' } as const; 
// type 
// {
//     readonly street: "Tokyo";
//     readonly country: "Japan";
// }


const user = { 
  email, 
  phones, 
  session,
  username, 
  roles, 
  address
} as const;
// {
//     readonly email: "john@gmail.com";
//     readonly phones: number[];
//     readonly session: {
//       id: number;
//       name: string;
//     };
//     readonly username: "John";
//     readonly roles: readonly ["read", "write"];
//     readonly address: {
//         readonly street: "Tokyo";
//         readonly country: "Japan";
//     };
// }


// With const assertion
// user.email = 'jim@gmail.com' // Error
// user.phones = []; // Error
user.phones.push(859324293);
// user.session = { name: 'new session name', id: 124 }; // Error
user.session.name = 'new session name';

// With const assertion + inner const assertion
// user.username = 'Jim'; // Error
// user.roles.push('ReadAndWrite'); // Error
// user.address.city = 'Osaka'; // Error

In addition, with const assertion help we can transform an array of strings into a string literal union type:

const roles = ['read', 'write', 'readAndWrite'] as const;

type Roles = typeof roles[number]; 
// equals to this
// type Roles = "read" | "write" | "readAndWrite"

// or even like this
type RolesInCapital = Capitalize<typeof roles[number]>;
// equals to this
// type RolesInCapital = "Read" | "Write" | "ReadAndWrite"

We use [number] after the roles array to tell TypeScript to grab all the numbered indexed values from the roles array.