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.

Practice

1. What does Object.entries(obj) return when used on an object obj?

easy

A. A new object with the same keys and values

B. An array of values only

C. An array of keys only

D. An array of [key, value] pairs from the object

Solution

1. Step 1: Understand Object.entries behavior

   Object.entries takes an object and returns an array where each element is a two-item array: the key and its corresponding value.

2. Step 2: Compare options to definition

   Only An array of [key, value] pairs from the object correctly describes this output as an array of [key, value] pairs. Options A, C, and D describe different outputs.

3. Final Answer:

   An array of [key, value] pairs from the object -> Option D

4. Quick Check:

   Object.entries = array of [key, value] pairs [OK]

Hint: Remember: entries give key-value pairs as arrays [OK]

Common Mistakes:

• Thinking it returns an object
• Confusing entries with keys or values only
• Expecting a flat array instead of pairs

2. Which of the following is the correct syntax to get entries from an object data?

easy

A. entries(Object.data)

B. Object.entries(data)

C. Object.entry(data)

D. data.entries()

Solution

1. Step 1: Recall the correct method name and usage

   The correct method is Object.entries() called with the object as argument.

2. Step 2: Check each option

   Object.entries(data) uses the correct syntax. data.entries() is invalid because objects don't have an entries() method. Object.entry(data) uses a wrong method name. entries(Object.data) has incorrect syntax.

3. Final Answer:

   Object.entries(data) -> Option B

4. Quick Check:

   Correct method is Object.entries(obj) [OK]

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

Common Mistakes:

• Calling entries as a method on the object
• Misspelling 'entries' as 'entry'
• Passing object incorrectly

3. What is the output of this code?

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

medium

A. ['a', 'b']

B. [['1', 'a'], ['2', 'b']]

C. [['a', 1], ['b', 2]]

D. [1, 2]

Solution

1. Step 1: Apply Object.entries to the object

   Object.entries returns an array of arrays, each inner array has the key and value from the object.

2. Step 2: Match output to options

   For obj = {a:1, b:2}, output is [['a', 1], ['b', 2]] which matches [['a', 1], ['b', 2]] exactly.

3. Final Answer:

   [['a', 1], ['b', 2]] -> Option C

4. Quick Check:

   Object.entries(obj) = [['a', 1], ['b', 2]] [OK]

Hint: Entries output is array of [key, value] pairs [OK]

Common Mistakes:

• Swapping keys and values
• Expecting a flat array of keys or values
• Confusing output format with JSON string

4. Find the error in this code snippet:

const user = {name: 'Alice', age: 25};
for (const [key, value] of user.entries()) {
  console.log(key, value);
}

medium

A. user.entries() is not a function

B. Missing curly braces in for loop

C. Destructuring syntax is incorrect

D. Console.log syntax error

Solution

1. Step 1: Identify method usage on object

   Plain objects do not have an entries() method; Object.entries() is a static method on Object.

2. Step 2: Correct usage

   To iterate entries, use for (const [key, value] of Object.entries(user)) instead of user.entries().

3. Final Answer:

   user.entries() is not a function -> Option A

4. Quick Check:

   Objects lack entries() method; use Object.entries(obj) [OK]

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

Common Mistakes:

• Calling entries() directly on object
• Confusing Object.entries with instance method
• Ignoring error message about missing function

5. Given const data = {x: 10, y: 0, z: null, w: 'hello'}, which code snippet correctly creates a new object with only entries having truthy values?

hard

A. Object.fromEntries(Object.entries(data).filter(([k, v]) => v))

B. Object.entries(data).filter(([k, v]) => v)

C. Object.entries(data).map(([k, v]) => v && [k, v])

D. Object.fromEntries(data.filter(([k, v]) => v))

Solution

1. Step 1: Filter entries with truthy values

   Use Object.entries(data) to get pairs, then filter pairs where value is truthy (non-zero, non-null, non-false).

2. Step 2: Convert filtered entries back to object

   Use Object.fromEntries() on filtered array to create new object with only truthy values.

3. Final Answer:

   Object.fromEntries(Object.entries(data).filter(([k, v]) => v)) -> Option A

4. Quick Check:

   Filter entries then fromEntries to object [OK]

Hint: Filter entries then use Object.fromEntries to rebuild object [OK]

Common Mistakes:

• Forgetting to convert back to object
• Using map instead of filter
• Trying to call filter on object directly