Array Insertion at End in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting elements at the end of the array?

Consider the following JavaScript code that inserts elements at the end of an array. What will be the final state of the array printed?

DSA Javascript

const arr = [1, 2, 3];
arr.push(4);
arr.push(5);
console.log(arr);

A [1, 2, 3, 5, 4]

B [4, 5, 1, 2, 3]

C [1, 2, 3]

D [1, 2, 3, 4, 5]

💡 Hint

Remember that push adds elements to the end of the array in the order they are called.

The push method adds elements to the end of the array in the order they are given. So after pushing 4 and then 5, the array becomes [1, 2, 3, 4, 5].

❓ Predict Output

intermediate

What is the output after using length property to insert at the end?

Look at this code snippet that inserts an element at the end of an array using the length property. What will be printed?

DSA Javascript

const arr = ['a', 'b', 'c'];
arr[arr.length] = 'd';
console.log(arr);

A ['a', 'b', 'c', 'd']

B ['d', 'a', 'b', 'c']

C ['a', 'b', 'c', undefined, 'd']

D ['a', 'b', 'c']

💡 Hint

Using arr.length as the index adds the element exactly at the end.

The length property gives the next available index at the end of the array. Assigning arr[arr.length] = 'd' adds 'd' at the end.

❓ Predict Output

advanced

What is the output after multiple insertions using push and length?

Analyze the following code that inserts elements at the end of an array using both push and length. What will be the final array printed?

DSA Javascript

const arr = [10];
arr.push(20);
arr[arr.length] = 30;
arr.push(40);
console.log(arr);

A [10, 20, 30]

B [10, 20, 30, 40]

C [10, 30, 20, 40]

D [10, 20, 40, 30]

💡 Hint

Both push and assigning at arr.length add elements at the end in order.

push(20) adds 20 at end: [10,20] arr[arr.length]=30 adds 30 at end: [10,20,30] push(40) adds 40 at end: [10,20,30,40]

❓ Predict Output

advanced

What error occurs when inserting at an invalid index?

What happens when you try to insert an element at an index beyond the array length like this?

DSA Javascript

const arr = [1, 2, 3];
arr[5] = 10;
console.log(arr);

A [1, 2, 3, undefined, undefined, 10]

B [1, 2, 3, 10]

C SyntaxError

D TypeError

💡 Hint

Assigning to an index beyond current length creates empty slots (undefined) in between.

JavaScript arrays are sparse. Assigning arr[5] = 10 creates undefined elements at indexes 3 and 4.

🧠 Conceptual

expert

Which method is most efficient for inserting multiple elements at the end of a large array?

You want to insert multiple elements at the end of a very large array efficiently. Which approach is best?

A Assign each element using arr[arr.length] = element in a loop.

B Use multiple push calls, one for each element.

C Use a single push call with all elements as arguments.

D Use unshift to add elements at the start repeatedly.

💡 Hint

Consider how many times the array resizes or shifts elements.

A single push call with all elements adds them at once, minimizing overhead. Multiple push calls or assignments cause repeated resizing. unshift adds at start, shifting all elements, which is inefficient.

Practice

1. What does the push() method do when used on a JavaScript array?

easy

A. Adds one or more elements to the end of the array

B. Removes the first element from the array

C. Sorts the array in ascending order

D. Reverses the order of elements in the array

Solution

1. Step 1: Understand the purpose of push()

   The push() method is designed to add elements to the end of an array.

2. Step 2: Compare with other array methods

   Other methods like shift() remove elements, sort() orders elements, and reverse() changes order, but push() only adds at the end.

3. Final Answer:

   Adds one or more elements to the end of the array -> Option A

4. Quick Check:

   push() adds elements at end [OK]

Hint: Remember: push adds at the end, pop removes from end [OK]

Common Mistakes:

• Confusing push() with pop() which removes elements
• Thinking push() adds elements at the start
• Mixing push() with sort() or reverse()

2. Which of the following is the correct syntax to add the element 5 at the end of array arr?

easy

A. arr.add(5);

B. arr.append(5);

C. arr.insertEnd(5);

D. arr.push(5);

Solution

1. Step 1: Recall JavaScript array methods

   JavaScript arrays use push() to add elements at the end.

2. Step 2: Check each option's validity

   add(), insertEnd(), and append() are not valid JavaScript array methods.

3. Final Answer:

   arr.push(5); -> Option D

4. Quick Check:

   Correct syntax for adding at end is push() [OK]

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

Common Mistakes:

• Using non-existent methods like add() or append()
• Confusing push() with unshift() which adds at start
• Missing parentheses in method call

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

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

medium

A. [1, 2, 3]

B. [4, 1, 2, 3]

C. [1, 2, 3, 4]

D. Error

Solution

1. Step 1: Understand initial array and push operation

   The array starts as [1, 2, 3]. The push(4) adds 4 at the end.

2. Step 2: Determine final array content

   After push, the array becomes [1, 2, 3, 4].

3. Final Answer:

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

4. Quick Check:

   push(4) adds 4 at end [OK]

Hint: push() adds element at end, so array grows rightwards [OK]

Common Mistakes:

• Thinking push adds element at start
• Expecting original array unchanged
• Confusing push with unshift

4. Identify the error in the following code snippet that tries to add 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 as a function

B. Array nums is not defined

C. push method does not exist on arrays

D. Missing semicolon after push

Solution

1. Step 1: Analyze the push usage

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

2. Step 2: Understand consequences of assignment

   This overwrites the push method with a number, so push no longer works as a function.

3. Final Answer:

   Using assignment instead of calling push as a function -> Option A

4. Quick Check:

   push must be called as a function, not assigned [OK]

Hint: Always call push() with parentheses to add elements [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 the code below, what will be the final state of arr?

let arr = [10];
arr.push(20);
arr.push(30, 40);
arr.push();

hard

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

B. [10, 20, 30, 40]

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

D. [10, 20, 30]

Solution

1. Step 1: Trace each push call

   Initial array: [10]. arr.push(20) adds 20 -> [10, 20]. arr.push(30, 40) adds both 30 and 40 -> [10, 20, 30, 40]. arr.push() with no arguments adds nothing.

2. Step 2: Confirm final array state

   No undefined or nested arrays are added. Final array is [10, 20, 30, 40].

3. Final Answer:

   [10, 20, 30, 40] -> Option B

4. Quick Check:

   push() adds all args at end; no args means no change [OK]

Hint: push() adds all arguments; no args means no change [OK]

Common Mistakes:

• Thinking push() with no args adds undefined
• Assuming push(30, 40) adds an array instead of separate elements
• Confusing nested arrays with multiple arguments