Modifying arrays in Javascript - Mini Project: Build & Apply

Modifying arrays

📖 Scenario: You are managing a small online store's product list. You want to update the list by adding new products and removing old ones.

🎯 Goal: Build a simple JavaScript program that starts with an initial array of products, adds a new product, removes a discontinued product, and then shows the updated list.

📋 What You'll Learn

Create an array called products with the exact values: 'apple', 'banana', 'carrot'

Create a variable called newProduct and set it to the string 'date'

Add the newProduct to the end of the products array

Remove the product 'banana' from the products array

Print the final products array

💡 Why This Matters

🌍 Real World

Managing product lists is common in online stores and inventory systems where items are added or removed regularly.

💼 Career

Knowing how to modify arrays is essential for frontend and backend developers working with lists of data.

1

Create the initial products array

Create an array called products with these exact values: 'apple', 'banana', 'carrot'

Javascript

# Create the products array with 'apple', 'banana', 'carrot'
// Your code here

Hint

Use square brackets [] to create an array and separate items with commas.

2

Add a new product variable

Create a variable called newProduct and set it to the string 'date'

Javascript

const products = ['apple', 'banana', 'carrot'];
// Create newProduct variable and set it to 'date'
// Your code here

Hint

Use const to create a variable and assign the string 'date'.

3

Modify the products array

Add the newProduct to the end of the products array using push, then remove the product 'banana' from the products array using splice

Javascript

const products = ['apple', 'banana', 'carrot'];
const newProduct = 'date';
// Add newProduct to products
// Remove 'banana' from products
// Your code here

Hint

Use push to add an item and splice with the index of 'banana' to remove it.

4

Display the updated products array

Write console.log(products) to print the updated products array

Javascript

const products = ['apple', 'banana', 'carrot'];
const newProduct = 'date';
products.push(newProduct);
const bananaIndex = products.indexOf('banana');
if (bananaIndex !== -1) {
    products.splice(bananaIndex, 1);
}
// Print the products array
// Your code here

Hint

Use console.log(products) to see the final list in the console.

Practice

1. Which method adds one or more elements to the end of a JavaScript array?

easy

A. push()

B. pop()

C. shift()

D. unshift()

Solution

1. Step 1: Understand array methods for adding elements

   push() adds elements to the end, unshift() adds to the start.

2. Step 2: Identify the method that adds to the end

   push() is the method that adds elements at the end of the array.

3. Final Answer:

   push() -> Option A

4. Quick Check:

   Adding at end = push() [OK]

Hint: push() adds at end, unshift() adds at start [OK]

Common Mistakes:

• Confusing push() with pop()
• Using shift() to add elements
• Thinking unshift() adds at end

2. Which of the following is the correct syntax to remove the first element from an array arr?

easy

A. arr.pop()

B. arr.shift()

C. arr.unshift()

D. arr.push()

Solution

1. Step 1: Recall methods that remove elements

   pop() removes from end, shift() removes from start.

2. Step 2: Identify method to remove first element

   shift() removes the first element of the array.

3. Final Answer:

   arr.shift() -> Option B

4. Quick Check:

   Remove first element = shift() [OK]

Hint: shift() removes first, pop() removes last [OK]

Common Mistakes:

• Using pop() to remove first element
• Using unshift() which adds elements
• Confusing push() with removal methods

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

let arr = [1, 2, 3, 4];
arr.splice(1, 2, 8, 9);
console.log(arr);

medium

A. [1, 8, 9, 4]

B. [1, 2, 3, 4]

C. [8, 9, 4]

D. [1, 8, 9]

Solution

1. Step 1: Understand splice parameters

   splice(1, 2, 8, 9) means start at index 1, remove 2 elements, then insert 8 and 9.

2. Step 2: Apply splice on array

   Original array: [1, 2, 3, 4]. Remove elements at index 1 and 2 (2 and 3), then insert 8 and 9 at index 1. Result: [1, 8, 9, 4].

3. Final Answer:

   [1, 8, 9, 4] -> Option A

4. Quick Check:

   splice removes and inserts = [1, 8, 9, 4] [OK]

Hint: splice(start, deleteCount, items...) edits array in place [OK]

Common Mistakes:

• Confusing splice with slice
• Not removing correct elements
• Forgetting splice inserts new items

4. The following code tries to remove the element '3' from the array but does not work as expected. What is the error?

let arr = [1, 2, 3, 4];
arr.splice(3, 1);
console.log(arr);

medium

A. The index 3 is out of range for removal

B. The splice method is used incorrectly; it should be pop()

C. The index 3 removes the wrong element, should be 2

D. splice cannot remove elements from arrays

Solution

1. Step 1: Identify the element to remove and its index

   The element '3' is at index 2 in the array [1, 2, 3, 4].

2. Step 2: Check the splice call

   The code calls arr.splice(3, 1), which removes the element at index 3 (which is 4), not 3.

3. Final Answer:

   The index 3 removes the wrong element, should be 2 -> Option C

4. Quick Check:

   Remove '3' at index 2, not 3 [OK]

Hint: Check array indexes carefully before splice [OK]

Common Mistakes:

• Using wrong index for splice
• Confusing pop() with splice()
• Thinking splice can't remove elements

5. You have an array arr = [5, 10, 15, 20]. You want to replace the middle two elements with 12 and 18 using a single method call. Which code correctly does this?

hard

A. arr.splice(2, 2, 12, 18);

B. arr.splice(0, 2, 12, 18);

C. arr.splice(1, 1, 12, 18);

D. arr.splice(1, 2, 12, 18);

Solution

1. Step 1: Identify indexes of middle two elements

   Array is [5, 10, 15, 20]. Middle two elements are at indexes 1 and 2 (10 and 15).

2. Step 2: Use splice to replace these elements

   Call arr.splice(1, 2, 12, 18) to start at index 1, remove 2 elements, and insert 12 and 18.

3. Final Answer:

   arr.splice(1, 2, 12, 18); -> Option D

4. Quick Check:

   Replace middle two with splice(1,2,12,18) [OK]

Hint: splice(startIndex, countToRemove, ...newItems) replaces items [OK]

Common Mistakes:

• Wrong start index
• Removing too few or too many elements
• Using splice with wrong parameters