TS18048: 'x' is possibly 'undefined'

Access properties safely on potentially null/undefined values.

interface User {
  name: string
  address?: {
    street: string
    city: string
  }
}

// ❌ address is possibly undefined
console.log(user.address.street)

// ✅ Optional chaining
console.log(user.address?.street ?? "No address")

// ✅ Chained optional access
const city = user?.address?.city ?? "Unknown"

Provide default values for possibly null/undefined expressions.

function greet(name?: string) {
  // ❌ name is possibly undefined
  console.log(`Hello, ${name.toUpperCase()}`)

  // ✅ Default value
  const safeName = name ?? "Guest"
  console.log(`Hello, ${safeName.toUpperCase()}`)
}

Check for null/undefined before accessing the value.

function processUser(user: User | undefined) {
  // ❌ user is possibly undefined
  console.log(user.name)

  // ✅ Type guard
  if (user) {
    console.log(user.name)  // TypeScript knows user is defined here
  }

  // ✅ Or throw if required
  if (!user) throw new Error("User required")
  console.log(user.name)  // Defined after the throw
}

Use ! to assert a value is not null/undefined (use sparingly).

// Only use when you are absolutely sure the value exists
function findUser(id: number): User | undefined {
  return users.find(u => u.id === id)
}

// We know this user exists (just created it)
const user = findUser(1)!
console.log(user.name)  // ✅ Non-null assertion