Object use cases in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using object property shorthand?

Consider the following JavaScript code snippet. What will be logged to the console?

Javascript

const name = 'Alice';
const age = 30;
const person = { name, age };
console.log(person);

A { name: 'Alice', age: 30 }

B { name: 'name', age: 'age' }

C { name: undefined, age: undefined }

D SyntaxError

💡 Hint

Object property shorthand lets you write { name, age } instead of { name: name, age: age }.

The object person uses property shorthand, so the keys are 'name' and 'age' with values from the variables name and age.

❓ Predict Output

intermediate

What is the output when accessing nested object properties?

What will this code output to the console?

Javascript

const data = { user: { id: 1, info: { name: 'Bob', city: 'NY' } } };
console.log(data.user.info.city);

A undefined

B 'NY'

C TypeError

D 'city'

💡 Hint

Access nested properties by chaining keys with dots.

data.user.info.city accesses the city property inside info inside user, which is 'NY'.

❓ Predict Output

advanced

What is the output of this code using Object.assign?

What will be logged to the console after running this code?

Javascript

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const result = Object.assign(target, source);
console.log(result);

A { a: 1, b: 4, c: 5 }

B { a: 1, b: 2, c: 5 }

C { b: 4, c: 5 }

D TypeError

💡 Hint

Object.assign copies properties from source to target, overwriting existing keys.

The property 'b' in target is overwritten by source's 'b', and 'c' is added.

❓ Predict Output

advanced

What error does this code raise when accessing a missing property?

What happens when this code runs?

Javascript

const obj = { x: 10 };
console.log(obj.y.z);

A ReferenceError

B undefined

C null

D TypeError: Cannot read property 'z' of undefined

💡 Hint

Accessing a property of undefined causes a TypeError.

obj.y is undefined, so trying to access .z on undefined throws a TypeError.

❓ Predict Output

expert

What is the output of this code using computed property names?

What will this code print to the console?

Javascript

const key = 'score';
const player = {
  [key]: 100,
  ['level' + 1]: 2
};
console.log(player);

A SyntaxError

B { score: 100, level: 1 }

C { score: 100, level1: 2 }

D { key: 100, level1: 2 }

💡 Hint

Square brackets let you use expressions as property names.

The property names are computed: 'score' and 'level1'.