Array Insertion at Beginning in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Array Insertion at Beginning' mean?

It means adding a new element to the start (first position) of an array, shifting all other elements one position to the right.

beginner

Which TypeScript method can be used to insert an element at the beginning of an array?

The unshift() method adds one or more elements to the start of an array and returns the new length of the array.

beginner

What happens to the existing elements when you insert at the beginning of an array?

All existing elements move one position to the right to make space for the new element at index 0.

beginner

Show a simple TypeScript code snippet to insert number 5 at the beginning of array [1, 2, 3].

const arr = [1, 2, 3];<br>arr.unshift(5);<br>// Now arr is [5, 1, 2, 3]

intermediate

Why might inserting at the beginning of an array be slower than inserting at the end?

Because all existing elements must be moved one position to the right, which takes more time as the array grows.

Which method adds an element to the start of an array in TypeScript?

A push()

B pop()

C unshift()

D shift()

unshift() adds elements at the beginning, push() adds at the end.

What happens to the existing elements when you insert at the beginning of an array?

A They stay in the same place

B They move one position to the right

C They move one position to the left

D They get deleted

Existing elements shift right to make space for the new element at index 0.

What is the output after this code?
const arr = [10, 20];
arr.unshift(5);
console.log(arr);

A [5, 20, 10]

B [10, 20, 5]

C [20, 10, 5]

D [5, 10, 20]

unshift(5) adds 5 at the start, so array becomes [5, 10, 20].

Which operation is generally slower for large arrays?

A Inserting at the beginning

B Accessing an element

C Inserting at the end

D Deleting the last element

Inserting at the beginning requires shifting all elements, which is slower.

What does the unshift() method return?

A The new length of the array

B The removed element

C The new array

D Undefined

unshift() returns the new length of the array after insertion.

Explain how inserting an element at the beginning of an array works and what happens to existing elements.

Think about how the array changes visually after insertion.

Write a TypeScript code snippet to insert the number 100 at the start of an array named numbers.

Use the method that adds elements at the beginning.

Practice

1. What does inserting an element at the beginning of an array do to the existing elements?

easy

A. Replaces the first element with the new element

B. Deletes the last element of the array

C. Shifts all existing elements one position to the right

D. Leaves the array unchanged

Solution

1. Step 1: Understand array insertion at beginning

   When a new element is inserted at the start, all existing elements move one step to the right to make space.

2. Step 2: Effect on existing elements

   This shifting means the first element moves to second position, second to third, and so on.

3. Final Answer:

   Shifts all existing elements one position to the right -> Option C

4. Quick Check:

   Insert at start shifts elements right [OK]

Hint: New start element pushes others right [OK]

Common Mistakes:

• Thinking insertion replaces first element
• Assuming array size decreases
• Believing array stays unchanged

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

easy

A. arr.unshift(5);

B. arr.addFirst(5);

C. arr.insert(0, 5);

D. arr.push(5);

Solution

1. Step 1: Recall array methods for insertion

   In TypeScript (JavaScript), `unshift` adds elements at the start of an array.

2. Step 2: Check other options

   `push` adds at the end, `insert` and `addFirst` are not valid array methods.

3. Final Answer:

   arr.unshift(5); -> Option A

4. Quick Check:

   Use unshift() to add at beginning [OK]

Hint: Use unshift() to add at start [OK]

Common Mistakes:

• Using push() which adds at the end
• Using non-existent methods like insert()
• Confusing unshift() with pop()

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

let arr = [10, 20, 30];
arr.unshift(5);
console.log(arr);

medium

A. [10, 20, 30, 5]

B. [5, 10, 20, 30]

C. [5, 20, 30]

D. [10, 5, 20, 30]

Solution

1. Step 1: Understand unshift effect

   unshift(5) adds 5 at the start, shifting existing elements right.

2. Step 2: Resulting array

   Original [10, 20, 30] becomes [5, 10, 20, 30].

3. Final Answer:

   [5, 10, 20, 30] -> Option B

4. Quick Check:

   unshift adds at front [OK]

Hint: unshift adds element at front [OK]

Common Mistakes:

• Thinking unshift adds at end
• Misplacing the new element in the array
• Confusing unshift with push

4. Identify the error in the following TypeScript code that tries to insert `100` at the beginning of array `nums`.

let nums = [1, 2, 3];
nums.add(0, 100);
console.log(nums);

medium

A. `add` is not a valid method for arrays

B. `console.log` is used incorrectly

C. Array 'nums' is not declared properly

D. The element 100 cannot be added to arrays

Solution

1. Step 1: Check array method usage

   Arrays in TypeScript do not have an `add` method; insertion at beginning uses `unshift`.

2. Step 2: Validate other parts

   Declaration and console.log are correct; adding 100 is allowed.

3. Final Answer:

   `add` is not a valid method for arrays -> Option A

4. Quick Check:

   Use unshift(), not add() [OK]

Hint: Use unshift(), not add() for insertion [OK]

Common Mistakes:

• Using non-existent add() method
• Thinking console.log is wrong
• Believing arrays can't hold numbers

5. Given an empty array `let data: number[] = [];`, which sequence of operations will result in the array `[3, 2, 1]` by inserting elements only at the beginning?

hard

A. data.unshift(1); data.unshift(2); data.unshift(3);

B. data.push(1); data.push(2); data.push(3);

C. data.push(3); data.unshift(2); data.push(1);

D. data.unshift(3); data.unshift(2); data.unshift(1);

Solution

1. Step 1: Understand insertion at beginning

   Each unshift adds element at front, pushing existing elements right.

2. Step 2: Trace operations

   Starting empty: unshift(3) -> [3], unshift(2) -> [2,3], unshift(1) -> [1,2,3].

3. Final Answer:

   data.unshift(3); data.unshift(2); data.unshift(1); -> Option D

4. Quick Check:

   Insert in reverse order with unshift [OK]

Hint: Insert last element first with unshift [OK]

Common Mistakes:

• Using push() which adds at end
• Inserting elements in wrong order
• Mixing push and unshift incorrectly