A quick demo of structural typing and JS object semantics in TS

structural-typing.ts

type User = { name: string, email: string }
type Admin = { name: string, role: string }

// Let's say hi to a user!
const greet1 = (u: User) => console.log(`Hi, ${u.name}`)

// What if we want to greet an Admin though... We can't re-use this
// What about taking a page out of C#'s book and use an interface?

interface INamed { name: string }
const greet2 = (n: INamed) => console.log(`Hi, ${n.name}`)

// Now it works on both! But we had to write a named interface JUST for this. Yuck.

// Let's try destructuring instead, perhaps a LITTLE repetitive...
// But very resilient to refactoring your program
const greet3 = ({ name }: { name: string }) => console.log(`Hi, ${name}!`)

const bob: User = { name: "Bob Bobbertson", email: "[email protected]" }
const ben: Admin = { name: "Ben", role: "Overlord" }

// We don't care about the extra info being passed, it doesn't matter
greet3(bob)
// Hi, Bob Bobbertson!

greet3(ben)
// Hi, Ben!

greet3({ name: 'Reginald' })
// Hi, Reginald!

const usersAndAdmins: (User | Admin)[] = 
  [
    {name: "Alice", email: "[email protected]"},
    bob,
    ben
  ]

// No code after this line knows about the types User and Admin


const generateUsername = ({ name } : { name: string }) => name.toLowerCase().replace(/s/g, '_') // From Big Boi to big_boi
const printId = ({ username } : { username: string }) => console.log(`Username: ${username}`)

usersAndAdmins
  .map(x => ({...x, username: generateUsername(x) }))
  .forEach(printId)
// alice
// bob_bobbertson
// ben