Object entries in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get an array of key-value pairs from the object.

Javascript

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

Drag options to blanks, or click blank then click option'

Aentries

Bentries()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Object.keys instead of Object.entries

Forgetting to call the method with parentheses

Use Object.entries(obj) to get an array of key-value pairs.

2fill in blank

medium

Complete the code to loop over the entries of the object.

Javascript

const obj = {x: 10, y: 20};
for (const [key, value] of Object.[1](obj)) {
  console.log(key, value);
}

Drag options to blanks, or click blank then click option'

AgetEntries

Bentries

Cvalues

Dkeys

Attempts:

3 left

💡 Hint

Common Mistakes

Using Object.keys which returns only keys

Using Object.values which returns only values

Use Object.entries(obj) to loop over key-value pairs.

3fill in blank

hard

Fix the error in the code to correctly get entries from the object.

Javascript

const obj = {name: 'Alice', age: 25};
const entries = Object.[1](obj);
console.log(entries);

Drag options to blanks, or click blank then click option'

Aentrys

Bentry

Centires

Dentries

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling the method name

Using singular form 'entry'

The correct method name is entries. Typos cause errors.

4fill in blank

hard

Fill both blanks to create an object from entries and log it.

Javascript

const entries = [['a', 1], ['b', 2]];
const obj = Object.[1](entries);
console.[2](obj);

Drag options to blanks, or click blank then click option'

AfromEntries

Blog

Cprint

Dentries

Attempts:

3 left

💡 Hint

Common Mistakes

Using Object.entries instead of fromEntries

Using console.print which is not a function

Use Object.fromEntries to create an object from entries and console.log to print it.

5fill in blank

hard

Fill all three blanks to filter entries with values greater than 10 and create a new object.

Javascript

const obj = {a: 5, b: 15, c: 25};
const filteredEntries = Object.entries(obj).filter(([[1], [2]]) => [3] > 10);
const newObj = Object.fromEntries(filteredEntries);
console.log(newObj);

Drag options to blanks, or click blank then click option'

Akey

Bvalue

Cval

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names in destructuring

Comparing key instead of value

Use key and value as variable names in destructuring, then filter by value > 10.

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