Object type annotation inline in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of inline object type annotation with optional property

What is the output of this TypeScript code when compiled and run with Node.js (ignoring type errors)?

Typescript

function greet(user: { name: string; age?: number }) {
  return `Hello, ${user.name}` + (user.age !== undefined ? `, age ${user.age}` : '');
}

console.log(greet({ name: 'Alice' }));

A "Hello, Alice, age 0"

B "Hello, Alice, age undefined"

C TypeError at runtime

D "Hello, Alice"

💡 Hint

Look at how the optional property age is checked before use.

The function checks if user.age is truthy before adding it to the string. Since age is undefined, it is skipped, so output is "Hello, Alice".

❓ Predict Output

intermediate

Output of inline object type annotation with nested object

What is the output of this TypeScript code?

Typescript

function getFullName(user: { name: { first: string; last: string } }) {
  return `${user.name.first} ${user.name.last}`;
}

console.log(getFullName({ name: { first: 'John', last: 'Doe' } }));

A "John Doe"

B "[object Object]"

C TypeError: Cannot read property 'first' of undefined

D "John undefined"

💡 Hint

Check how nested object properties are accessed.

The function accesses first and last properties correctly from the nested name object, so it returns "John Doe".

🔧 Debug

advanced

Identify the error in inline object type annotation

What error will this TypeScript code produce?

Typescript

function printUser(user: { name: string; age: number }) {
  console.log(`Name: ${user.name}, Age: ${user.age}`);
}

printUser({ name: 'Bob' });

A Error: Property 'age' is missing in type '{ name: string; }' but required in type '{ name: string; age: number; }'.

B Runtime error: Cannot read property 'age' of undefined

C No error, prints: Name: Bob, Age: undefined

D SyntaxError: Unexpected token

💡 Hint

Check if all required properties are provided in the argument.

The age property is required but missing in the argument object, so TypeScript reports a compile-time error.

❓ Predict Output

advanced

Output of inline object type annotation with readonly property

What is the output of this TypeScript code?

Typescript

function updateUser(user: { readonly id: number; name: string }) {
  // user.id = 10; // Uncommenting this line causes error
  return `User ${user.id}: ${user.name}`;
}

console.log(updateUser({ id: 5, name: 'Eve' }));

A "User undefined: Eve"

B TypeError: Cannot assign to read only property 'id'

C "User 5: Eve"

D SyntaxError: Unexpected token

💡 Hint

Readonly means the property cannot be changed after creation.

The code does not try to change the readonly property, so it prints "User 5: Eve" without error.

❓ Predict Output

expert

Output of inline object type annotation with index signature

What is the output of this TypeScript code?

Typescript

function sumScores(scores: { [key: string]: number }) {
  let total = 0;
  for (const key in scores) {
    total += scores[key];
  }
  return total;
}

console.log(sumScores({ math: 10, science: 20, english: 15 }));

A TypeError: scores[key] is not a number

B 45

C NaN

D SyntaxError: Unexpected token

💡 Hint

Index signature allows any string key with number values.

The function sums all values in the object and returns 45.