Object keys and values in Javascript

Introduction

We use object keys and values to store and organize information in pairs, like labels and their details.

When you want to store information about a person, like name and age.

When you need to group related data, like settings for an app.

When you want to look up a value quickly using a label.

When you want to change or add information easily by key.

Syntax

Javascript

const obj = {
  key1: 'value1',
  key2: 'value2'
};

// To get keys:
Object.keys(obj);

// To get values:
Object.values(obj);

Keys are always strings or symbols, values can be any type.

Object.keys() returns an array of keys, Object.values() returns an array of values.

Examples

This shows keys ['name', 'age'] and values ['Alice', 30].

Javascript

const person = {
  name: 'Alice',
  age: 30
};

console.log(Object.keys(person));
console.log(Object.values(person));

Gets keys ['darkMode', 'fontSize'] and values [true, 16].

Javascript

const settings = {
  darkMode: true,
  fontSize: 16
};

const keys = Object.keys(settings);
const values = Object.values(settings);

console.log(keys);
console.log(values);

Sample Program

This program creates a car object and prints its keys and values.

Javascript

const car = {
  brand: 'Toyota',
  year: 2022,
  color: 'red'
};

console.log('Keys:', Object.keys(car));
console.log('Values:', Object.values(car));

Important Notes

Keys order is the same as they were added in most cases.

Values can be any type: strings, numbers, booleans, objects, etc.

Summary

Objects store data in key-value pairs.

Use Object.keys() to get all keys as an array.

Use Object.values() to get all values as an array.

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