Array Insertion at End in DSA Typescript

Mental Model

Adding a new item to the end of a list means placing it right after the last item.

Analogy: Imagine a line of people waiting to buy tickets. When a new person arrives, they stand at the very end of the line.

[1] -> [2] -> [3] -> null
↑ array start

Dry Run Walkthrough

Input: array: [1, 2, 3], insert value 4 at end

Goal: Add the value 4 to the end of the array so it becomes [1, 2, 3, 4]

Step 1: Check current length of array (3)

[1] -> [2] -> [3] -> null
length = 3

Why: We need to know where the end is to add the new value

Step 2: Place value 4 at index 3 (end)

[1] -> [2] -> [3] -> [4] -> null

Why: Adding at the position equal to current length appends the value at the end

Step 3: Update array length to 4

[1] -> [2] -> [3] -> [4] -> null
length = 4

Why: The array now includes the new value, so length increases by one

Result:

[1] -> [2] -> [3] -> [4] -> null

Annotated Code

DSA Typescript

class ArrayInsertion {
  arr: number[];

  constructor(initial: number[]) {
    this.arr = initial;
  }

  insertAtEnd(value: number): void {
    // Place value at the index equal to current length
    this.arr[this.arr.length] = value;
  }

  printArray(): void {
    let output = '';
    for (let i = 0; i < this.arr.length; i++) {
      output += this.arr[i] + (i < this.arr.length - 1 ? ' -> ' : ' -> null');
    }
    console.log(output);
  }
}

// Driver code
const arrayInsertion = new ArrayInsertion([1, 2, 3]);
arrayInsertion.insertAtEnd(4);
arrayInsertion.printArray();

this.arr[this.arr.length] = value;

Add new value at the position after the last element

Output

1 -> 2 -> 3 -> 4 -> null

Complexity Analysis

Time: O(1) because adding at the end uses the known length index directly

Space: O(1) because no extra space is needed besides the new element

vs Alternative: Compared to inserting at the start which requires shifting all elements (O(n)), this is faster and simpler

Edge Cases

empty array

The value is placed at index 0, making the array have one element

DSA Typescript

this.arr[this.arr.length] = value;

array with one element

The value is added at index 1, extending the array

DSA Typescript

this.arr[this.arr.length] = value;

When to Use This Pattern

When you need to add an item to the end of a list quickly, use array insertion at end because it uses the length as the index.

Common Mistakes

Mistake: Trying to insert at index length - 1 which overwrites the last element
Fix: Insert at index equal to the current length to add after the last element

Summary

Adds a new value to the end of an array.

Use when you want to append data without changing existing elements.

The key is to use the array's length as the insertion index.

Practice

1. What does inserting an element at the end of an array do?

easy

A. Adds the element as the last item in the array

B. Removes the first element of the array

C. Replaces the first element with the new element

D. Sorts the array in ascending order

Solution

1. Step 1: Understand array insertion

   Inserting at the end means adding a new element after the last existing element.

2. Step 2: Effect on array

   The new element becomes the last item, increasing the array size by one.

3. Final Answer:

   Adds the element as the last item in the array -> Option A

4. Quick Check:

   Insertion at end = append last [OK]

Hint: Insertion at end means append after last element [OK]

Common Mistakes:

• Confusing insertion at end with removal
• Thinking insertion replaces elements
• Assuming insertion sorts the array

2. Which of the following is the correct TypeScript syntax to insert an element `5` at the end of array `arr`?

easy

A. arr.insert(5);

B. arr.add(5);

C. arr.push(5);

D. arr.append(5);

Solution

1. Step 1: Identify correct method for insertion at end

   In TypeScript (JavaScript), the method to add at the end is `push()`.

2. Step 2: Check other options

   Methods `add()`, `insert()`, and `append()` do not exist on arrays in TypeScript.

3. Final Answer:

   arr.push(5); -> Option C

4. Quick Check:

   Use push() to add at end [OK]

Hint: Use push() method to add at array end [OK]

Common Mistakes:

• Using non-existent methods like add or append
• Confusing push() with pop()
• Trying to assign directly to arr[arr.length]

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

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

medium

A. [4, 1, 2, 3]

B. [1, 2, 3, 4]

C. [1, 2, 3]

D. [1, 2, 4, 3]

Solution

1. Step 1: Initial array state

   Array starts as [1, 2, 3].

2. Step 2: Apply push(4)

   push(4) adds 4 at the end, so array becomes [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option B

4. Quick Check:

   push adds element at end [OK]

Hint: push() adds element at array end [OK]

Common Mistakes:

• Thinking push adds at start
• Confusing push with unshift
• Expecting array to remain unchanged

4. Identify the error in the following TypeScript code that tries to insert `10` at the end of array `nums`.

let nums = [1, 2, 3];
nums.push = 10;
console.log(nums);

medium

A. Using assignment instead of calling push method

B. Array nums is not declared properly

C. push method does not exist on arrays

D. Console.log syntax is incorrect

Solution

1. Step 1: Analyze push usage

   The code assigns 10 to nums.push instead of calling nums.push(10).

2. Step 2: Effect of assignment

   This overwrites the push method, so no insertion happens and push is lost.

3. Final Answer:

   Using assignment instead of calling push method -> Option A

4. Quick Check:

   push must be called as a function [OK]

Hint: Call push() as a function, don't assign to it [OK]

Common Mistakes:

• Assigning value to push instead of calling it
• Forgetting parentheses after push
• Assuming push is a property, not a method

5. Given an empty array `let data: number[] = [];`, which code snippet correctly inserts numbers 1 to 3 at the end of the array using a loop?

hard

A. for(let i = 0; i < 3; i++) { data.push(i); }

B. for(let i = 1; i <= 3; i++) { data[i] = i; }

C. for(let i = 1; i < 3; i++) { data.push(i); }

D. for(let i = 1; i <= 3; i++) { data.push(i); }

Solution

1. Step 1: Understand loop range and insertion

   We want to insert 1, 2, 3 at the end, so loop from 1 to 3 inclusive.

2. Step 2: Check each option

   A pushes 0,1,2 instead of 1,2,3.
   B assigns directly to indices 1,2,3 leaving index 0 empty.
   C uses push with i from 1 to 3, correctly adding 1,2,3.
   D loops from 1 to 2 only, missing 3.

3. Final Answer:

   for(let i = 1; i <= 3; i++) { data.push(i); } -> Option D

4. Quick Check:

   Loop 1 to 3 with push() adds correct elements [OK]

Hint: Loop from 1 to 3 and push each value [OK]

Common Mistakes:

• Using wrong loop range
• Assigning to array indices instead of push
• Starting loop at 0 instead of 1