TS2345: Argument of type 'X' is not assignable to parameter of type 'Y'

What’s happening?

This error occurs when you pass an argument to a function that doesn’t match the expected parameter type.

---

Reproduction

function greet(name: string) {
  console.log(`Hello, ${name}!`);
}

const userName: string | undefined = getUserName();
greet(userName);
// ❌ Argument of type 'string | undefined' is not assignable to parameter of type 'string'

---

Solutions

Option 1: Narrow the type before calling

const userName = getUserName();
if (userName !== undefined) {
  greet(userName); // ✅
}

Option 2: Provide a default value

greet(userName ?? "Guest"); // ✅

Option 3: Update the function signature

function greet(name: string | undefined) {
  console.log(`Hello, ${name ?? "Guest"}!`);
}

greet(userName); // ✅

---

Common scenarios

Passing optional props

function setUserId(id: number) { /* ... */ }

interface Props {
  userId?: number;
}

function Component({ userId }: Props) {
  setUserId(userId);
  // ❌ Argument of type 'number | undefined' is not assignable

  // ✅ Fix:
  if (userId !== undefined) {
    setUserId(userId);
  }
}

---

Key takeaways

• Always check the function signature
• Use type narrowing or defaults before calling
• Consider making function parameters more flexible

---

Additional resources

• Function Parameter Types