Object keys and values in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Object.keys and Object.values

What is the output of the following code?

Javascript

const obj = {a: 1, b: 2, c: 3};
console.log(Object.keys(obj));
console.log(Object.values(obj));

A ["a", "b", "c"] and [1, 2, 3]

B [1, 2, 3] and ["a", "b", "c"]

C ["a", "b", "c"] and ["1", "2", "3"]

D ["1", "2", "3"] and [1, 2, 3]

💡 Hint

Object.keys returns an array of property names, Object.values returns an array of property values.

Object.keys(obj) returns an array of the object's keys as strings. Object.values(obj) returns an array of the values in the same order.

❓ Predict Output

intermediate

Output of Object.keys with non-enumerable property

What will be logged to the console?

Javascript

const obj = {};
Object.defineProperty(obj, 'hidden', {value: 42, enumerable: false});
obj.visible = 7;
console.log(Object.keys(obj));

A ["hidden", "visible"]

B [42, 7]

C ["visible"]

D []

💡 Hint

Object.keys only lists enumerable properties.

The property 'hidden' is non-enumerable, so Object.keys only returns ['visible'].

❓ Predict Output

advanced

Output of Object.entries with nested objects

What is the output of this code?

Javascript

const obj = {x: 10, y: {z: 20}};
console.log(Object.entries(obj));

A [["x", 10], ["y", "{z: 20}"]]

B [["x", 10], ["z", 20]]

C ["x", 10, "y", {z: 20}]

D [["x", 10], ["y", {z: 20}]]

💡 Hint

Object.entries returns an array of [key, value] pairs, values can be objects.

Object.entries returns an array of arrays, each with key and value. The value can be an object itself.

❓ Predict Output

advanced

Output of Object.keys on array

What will this code output?

Javascript

const arr = ['a', 'b', 'c'];
console.log(Object.keys(arr));

A ["0", "1", "2"]

B [0, 1, 2]

C ["a", "b", "c"]

D []

💡 Hint

Arrays are objects with numeric keys as strings.

Object.keys on an array returns the indices as strings: ['0', '1', '2'].

❓ Predict Output

expert

Output of Object.values with symbol keys

What is the output of this code?

Javascript

const sym = Symbol('id');
const obj = {a: 1, [sym]: 2};
console.log(Object.values(obj));

A [1, 2]

B [1]

C [2]

D []

💡 Hint

Symbol keys are not included in Object.values.

Object.values only returns values of string-keyed enumerable properties, symbol keys are ignored.