Array Insertion at Middle Index in DSA Javascript - Build from Scratch

Array Insertion at Middle Index

📖 Scenario: You are managing a list of daily tasks. Sometimes, you need to add a new task right in the middle of your current list to keep things balanced.

🎯 Goal: Learn how to insert a new item exactly in the middle of an array in JavaScript.

📋 What You'll Learn

Create an array called tasks with exact string values

Create a variable called newTask with a specific string

Calculate the middle index of the tasks array

Insert newTask into tasks at the middle index

Print the updated tasks array

💡 Why This Matters

🌍 Real World

In daily task management apps, inserting tasks in the middle helps keep priorities balanced and organized.

💼 Career

Understanding array manipulation is essential for software developers working with lists, queues, and dynamic data.

1

Create the initial tasks array

Create an array called tasks with these exact string values: 'Wash dishes', 'Do laundry', 'Buy groceries', 'Clean room'.

DSA Javascript

// Create the tasks array with the given tasks
// Your code here

Hint

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

2

Create the new task to insert

Create a variable called newTask and set it to the string 'Pay bills'.

DSA Javascript

const tasks = ['Wash dishes', 'Do laundry', 'Buy groceries', 'Clean room'];
// Create the newTask variable with the value 'Pay bills'
// Your code here

Hint

Use const to declare newTask and assign the string 'Pay bills'.

3

Calculate the middle index and insert the new task

Calculate the middle index of the tasks array using Math.floor(tasks.length / 2) and store it in a variable called middleIndex. Then insert newTask into tasks at middleIndex using tasks.splice(middleIndex, 0, newTask).

DSA Javascript

const tasks = ['Wash dishes', 'Do laundry', 'Buy groceries', 'Clean room'];
const newTask = 'Pay bills';
// Calculate middleIndex and insert newTask into tasks
// Your code here

Hint

Use Math.floor to get the middle index and splice to insert without removing any items.

4

Print the updated tasks array

Print the tasks array using console.log(tasks) to see the updated list with the new task inserted in the middle.

DSA Javascript

const tasks = ['Wash dishes', 'Do laundry', 'Buy groceries', 'Clean room'];
const newTask = 'Pay bills';
const middleIndex = Math.floor(tasks.length / 2);
tasks.splice(middleIndex, 0, newTask);
// Print the tasks array
// Your code here

Hint

Use console.log to display the array in the console.

Practice

1. What does inserting an element at the middle index of an array mean?

easy

A. Adding the element exactly in the center position of the array

B. Adding the element at the start of the array

C. Adding the element at the end of the array

D. Replacing the first element with the new element

Solution

1. Step 1: Understand array indexing

   Arrays have positions starting from 0 to length-1. The middle index is roughly length/2.

2. Step 2: Define insertion at middle

   Inserting at middle means placing the new element at the middle index, shifting others right.

3. Final Answer:

   Adding the element exactly in the center position of the array -> Option A

4. Quick Check:

   Middle insertion = center position [OK]

Hint: Middle index is array length divided by 2 [OK]

Common Mistakes:

• Confusing middle with start or end
• Replacing element instead of inserting
• Not shifting elements after insertion

2. Which of the following is the correct syntax to insert an element 'x' at the middle index of array 'arr' in JavaScript?

easy

A. arr.insert(Math.floor(arr.length / 2), x);

B. arr.add(Math.floor(arr.length / 2), x);

C. arr.splice(Math.floor(arr.length / 2), 0, x);

D. arr.push(Math.floor(arr.length / 2), x);

Solution

1. Step 1: Identify JavaScript method for insertion

   JavaScript arrays use splice(start, deleteCount, items...) to insert elements.

2. Step 2: Calculate middle index and insert

   Using Math.floor(arr.length / 2) gives middle index; splice with 0 deletes nothing and inserts 'x'.

3. Final Answer:

   arr.splice(Math.floor(arr.length / 2), 0, x); -> Option C

4. Quick Check:

   splice + middle index + 0 delete = insert [OK]

Hint: Use splice with 0 delete count to insert [OK]

Common Mistakes:

• Using non-existent insert or add methods
• Using push which adds only at end
• Not calculating middle index correctly

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

const arr = [10, 20, 30, 40];
arr.splice(Math.floor(arr.length / 2), 0, 25);
console.log(arr);

medium

A. [10, 20, 25, 30, 40]

B. [10, 20, 30, 25, 40]

C. [25, 10, 20, 30, 40]

D. [10, 20, 30, 40, 25]

Solution

1. Step 1: Calculate middle index

   arr.length = 4, middle index = Math.floor(4/2) = 2.

2. Step 2: Insert 25 at index 2

   Using splice(2, 0, 25) inserts 25 at index 2, shifting elements right.

3. Final Answer:

   [10, 20, 25, 30, 40] -> Option A

4. Quick Check:

   Insert at index 2 shifts right [OK]

Hint: Middle index = floor(length/2), splice inserts at that index [OK]

Common Mistakes:

• Inserting at wrong index
• Appending instead of inserting
• Misunderstanding splice parameters

4. Identify the error in the following code that tries to insert 15 at the middle of array 'nums':

const nums = [5, 10, 20, 25];
nums.insert(nums.length / 2, 15);
console.log(nums);

medium

A. The middle index calculation is incorrect

B. 'insert' is not a valid array method in JavaScript

C. The element 15 should be added using push()

D. The array should be declared with let instead of const

Solution

1. Step 1: Check array method used

   JavaScript arrays do not have an 'insert' method; splice should be used instead.

2. Step 2: Confirm middle index calculation

   nums.length / 2 = 2 is correct, but method is invalid causing error.

3. Final Answer:

   'insert' is not a valid array method in JavaScript -> Option B

4. Quick Check:

   Use splice, not insert [OK]

Hint: Remember: JavaScript arrays use splice, not insert [OK]

Common Mistakes:

• Using non-existent insert method
• Ignoring method errors
• Confusing push with insert

5. Given an array arr = [1, 3, 5, 7, 9], write code to insert the number 4 exactly in the middle so that the array remains sorted. What will be the final array?

hard

A. [1, 3, 5, 7, 9, 4]

B. [1, 3, 5, 4, 7, 9]

C. [4, 1, 3, 5, 7, 9]

D. [1, 3, 4, 5, 7, 9]

Solution

1. Step 1: Find middle index for insertion

   arr.length = 5, middle index = Math.floor(5 / 2) = 2.

2. Step 2: Insert 4 at index 2 using splice

   arr.splice(2, 0, 4) inserts 4 before element 5, keeping array sorted.

3. Final Answer:

   [1, 3, 4, 5, 7, 9] -> Option D

4. Quick Check:

   Insert 4 at index 2 keeps order [OK]

Hint: Insert at floor(length/2) to keep sorted order [OK]

Common Mistakes:

• Inserting at wrong index breaking order
• Appending instead of inserting
• Not using splice correctly