Object entries in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of Object.entries on a simple object?

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

Javascript

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

A [['a', 1], ['b', 2], ['c', 3]]

B [['1', 'a'], ['2', 'b'], ['3', 'c']]

C [['a', '1'], ['b', '2'], ['c', '3']]

D [['a', 1, 'b', 2, 'c', 3]]

💡 Hint

Object.entries returns an array of [key, value] pairs.

Object.entries returns an array where each element is a two-item array: the first is the key as a string, the second is the value. So for {a:1, b:2, c:3}, it returns [['a',1], ['b',2], ['c',3]].

❓ Predict Output

intermediate

What does Object.entries return for an object with symbol keys?

What will be the output of the following code?

Javascript

const sym = Symbol('id');
const obj = {a: 10, [sym]: 20};
console.log(Object.entries(obj));

A []

B [['a', 10], [Symbol(id), 20]]

C [['a', 10], ['sym', 20]]

D [['a', 10]]

💡 Hint

Object.entries only includes string keys, not symbol keys.

Object.entries returns only enumerable string-keyed properties. Symbol keys are ignored, so only [['a', 10]] is returned.

❓ Predict Output

advanced

Output of Object.entries on an object with inherited properties

What will this code print?

Javascript

const parent = {x: 1};
const child = Object.create(parent);
child.y = 2;
console.log(Object.entries(child));

A [['y', 2]]

B []

C [['x', 1], ['y', 2]]

D [['x', 1]]

💡 Hint

Object.entries only lists own enumerable properties, not inherited ones.

Object.entries returns only the object's own enumerable properties. 'y' is own property, 'x' is inherited, so only [['y', 2]] is returned.

❓ Predict Output

advanced

What is the output of Object.entries on an array?

What will this code print?

Javascript

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

A [['a', 0], ['b', 1], ['c', 2]]

B [['0', 'a'], ['1', 'b'], ['2', 'c']]

C [['length', 3]]

D []

💡 Hint

Arrays are objects with keys as string indices.

Arrays are objects with keys as string indices. Object.entries returns these index-value pairs as [['0','a'], ['1','b'], ['2','c']].

🧠 Conceptual

expert

Which option causes a TypeError when using Object.entries?

Which of the following code snippets will cause a TypeError when calling Object.entries?

A Object.entries([1,2,3])

B Object.entries({a:1})

C Object.entries(null)

D Object.entries('abc')

💡 Hint

Object.entries expects an object, null is not an object.

Object.entries throws a TypeError if the argument is null or undefined because they are not objects. Other options are valid inputs.