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.

Practice

1. Which method returns an array of all the keys in a JavaScript object?
const obj = {a: 1, b: 2};

easy

A. Object.values(obj)

B. Object.entries(obj)

C. Object.keys(obj)

D. Object.get(obj)

Solution

1. Step 1: Understand Object.keys()

   Object.keys() returns an array of the object's own enumerable property names (keys).

2. Step 2: Compare with other methods

   Object.values() returns values, Object.entries() returns key-value pairs, Object.get() is not a valid method.

3. Final Answer:

   Object.keys(obj) -> Option C

4. Quick Check:

   Keys = Object.keys(obj) [OK]

Hint: Keys come from Object.keys(), values from Object.values() [OK]

Common Mistakes:

• Confusing Object.keys() with Object.values()
• Using Object.entries() to get only keys
• Trying to use Object.get() which doesn't exist

2. Which of the following is the correct syntax to get all values from an object const obj = {x: 10, y: 20};?

easy

A. Object.values(obj)

B. obj.values()

C. Object.keys(obj)

D. Object.getValues(obj)

Solution

1. Step 1: Identify the method for values

   Object.values(obj) returns an array of the object's values.

2. Step 2: Check syntax validity

   obj.values() and Object.getValues() are not valid JavaScript methods.

3. Final Answer:

   Object.values(obj) -> Option A

4. Quick Check:

   Values = Object.values(obj) [OK]

Hint: Use Object.values() to get all values from an object [OK]

Common Mistakes:

• Using Object.keys() instead of Object.values()
• Calling values() directly on the object
• Using non-existent Object.getValues() method

3. What will be the output of the following code?

const obj = {a: 5, b: 10};
console.log(Object.keys(obj));

medium

A. ['a', 'b']

B. ['5', '10']

C. [5, 10]

D. ['obj', 'keys']

Solution

1. Step 1: Understand Object.keys() output

   Object.keys(obj) returns an array of the keys, which are 'a' and 'b'.

2. Step 2: Check the console.log output

   The console will print ['a', 'b'] as the keys array.

3. Final Answer:

   ['a', 'b'] -> Option A

4. Quick Check:

   Keys = ['a', 'b'] [OK]

Hint: Object.keys() returns keys as strings in an array [OK]

Common Mistakes:

• Confusing keys with values
• Expecting numeric values instead of string keys
• Misreading the output as property values

4. Find the error in the following code:

const data = {name: 'Alice', age: 25};
const keys = data.keys();
console.log(keys);

medium

A. No error, code works fine

B. data.keys() returns undefined

C. data.keys() returns an array of values

D. data.keys() is not a function

Solution

1. Step 1: Check method existence on object

   Plain objects do not have a keys() method; Object.keys() is a static method.

2. Step 2: Identify the error type

   Calling data.keys() causes a TypeError: data.keys is not a function.

3. Final Answer:

   data.keys() is not a function -> Option D

4. Quick Check:

   data.keys() causes TypeError [OK]

Hint: Use Object.keys(obj), not obj.keys() [OK]

Common Mistakes:

• Calling keys() directly on object instance
• Confusing Object.keys() with instance method
• Expecting keys() to return values

5. Given the object const scores = {alice: 10, bob: 15, charlie: 0, dave: null};, which code snippet correctly creates an array of keys whose values are truthy?

hard

A. Object.values(scores).filter(value => value)

B. Object.keys(scores).filter(key => scores[key])

C. Object.entries(scores).filter(([k,v]) => k).map(([k,v]) => v)

D. Object.keys(scores).map(key => scores[key])

Solution

1. Step 1: Understand filtering keys by truthy values

   We want keys where their values are truthy (non-zero, non-null).

2. Step 2: Analyze each option

   Object.keys(scores).filter(key => scores[key]) filters keys by checking if scores[key] is truthy, returning keys correctly.
   Object.values(scores).filter(value => value) returns values, not keys.
   Object.entries(scores).filter(([k,v]) => k).map(([k,v]) => v) filters by keys (always truthy) and maps to values, wrong.
   Object.keys(scores).map(key => scores[key]) maps keys to values without filtering.

3. Final Answer:

   Object.keys(scores).filter(key => scores[key]) -> Option B

4. Quick Check:

   Filter keys by truthy values = Object.keys(scores).filter(key => scores[key]) [OK]

Hint: Filter keys by checking their values truthiness [OK]

Common Mistakes:

• Filtering values but expecting keys
• Mapping keys to values without filtering
• Filtering keys by keys instead of values