Array Access and Update at Index in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to access an element in an array by index?

Accessing an element by index means retrieving the value stored at a specific position in the array, where the position is given by the index number starting from 0.

beginner

How do you update an element at a specific index in an array?

You update an element by assigning a new value to the array at that index, for example: array[index] = newValue.

beginner

What happens if you try to access an index outside the array length?

Accessing an index outside the array length returns undefined in JavaScript, meaning there is no value at that position.

beginner

Explain the difference between accessing and updating an array element.

Accessing means reading the value at a given index without changing it. Updating means changing the value at that index to something new.

beginner

In JavaScript, what is the index of the first element in an array?

The index of the first element in a JavaScript array is 0.

What is the result of accessing array[2] if array = [10, 20, 30, 40]?

A 30

B 20

C 40

D undefined

Index 2 corresponds to the third element, which is 30.

How do you update the first element of an array named 'nums' to 100?

A nums[0] = 100;

B nums[1] = 100;

C nums.update(0, 100);

D nums.set(1, 100);

The first element is at index 0, so nums[0] = 100 updates it.

What happens if you access array[5] when array length is 3?

A Returns last element

B Returns undefined

C Throws an error

D Returns null

Accessing an index beyond the array length returns undefined in JavaScript.

Which index corresponds to the last element in an array of length 7?

A 0

B 7

C 1

D 6

Array indices start at 0, so the last element is at length - 1, which is 6.

If you want to read the value at index 3 without changing it, what operation do you perform?

A Delete array[3]

B Update array[3]

C Access array[3]

D Insert at array[3]

Accessing means reading the value at that index without changing it.

Describe how to access and update an element at a specific index in an array.

Think about how you get and set values using the array name and index.

What happens if you try to access an index that is outside the current array length in JavaScript?

Consider what JavaScript returns when you ask for a position that does not exist.

Practice

1. What does arr[2] return if arr = [10, 20, 30, 40]?

easy

A. 40

B. 20

C. 10

D. 30

Solution

1. Step 1: Understand array indexing

   Arrays start at index 0, so index 2 is the third element.

2. Step 2: Identify the element at index 2

   The element at index 2 in [10, 20, 30, 40] is 30.

3. Final Answer:

   30 -> Option D

4. Quick Check:

   Index 2 element = 30 [OK]

Hint: Remember arrays start at zero index [OK]

Common Mistakes:

• Thinking index starts at 1
• Confusing element value with index
• Using negative index without explanation

2. Which of the following is the correct way to update the 3rd element of array nums to 50?

easy

A. nums[3] = 50;

B. nums(3) = 50;

C. nums[2] = 50;

D. nums{2} = 50;

Solution

1. Step 1: Recall array indexing syntax

   In JavaScript, arrays use square brackets with zero-based index.

2. Step 2: Identify correct index for 3rd element

   The 3rd element is at index 2, so nums[2] = 50; is correct.

3. Final Answer:

   nums[2] = 50; -> Option C

4. Quick Check:

   Use square brackets and zero-based index [OK]

Hint: Use square brackets and zero-based index [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based index
• Using curly braces for arrays

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

let arr = [5, 10, 15, 20];
arr[1] = arr[3];
console.log(arr);

medium

A. [5, 20, 15, 20]

B. [5, 10, 15, 20]

C. [20, 10, 15, 20]

D. [5, 15, 15, 20]

Solution

1. Step 1: Understand the assignment operation

   The element at index 1 is set to the value at index 3, which is 20.

2. Step 2: Update the array and print

   Original array: [5, 10, 15, 20]. After update: [5, 20, 15, 20].

3. Final Answer:

   [5, 20, 15, 20] -> Option A

4. Quick Check:

   arr[1] = arr[3] updates second element to 20 [OK]

Hint: Remember assignment copies value at index [OK]

Common Mistakes:

• Confusing indexes
• Expecting arr[3] to change
• Thinking assignment swaps values

4. Find the error in the following code snippet:

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

medium

A. No error, array expands and adds 4 at index 3

B. Array becomes empty after assignment

C. Syntax error due to invalid index

D. Index 3 is out of bounds, causes runtime error

Solution

1. Step 1: Understand JavaScript array behavior on out-of-range index

   Assigning to index 3 adds a new element; arrays auto-expand.

2. Step 2: Check the resulting array

   Original: [1, 2, 3]. After assignment: [1, 2, 3, 4]. No error occurs.

3. Final Answer:

   No error, array expands and adds 4 at index 3 -> Option A

4. Quick Check:

   JS arrays auto-expand on assignment [OK]

Hint: JS arrays grow when assigned out-of-range index [OK]

Common Mistakes:

• Expecting runtime error
• Thinking syntax error occurs
• Assuming array clears on assignment

5. Given let arr = [2, 4, 6, 8, 10]; write code to double the value at index 2 and then swap it with the value at index 4. What is the final array?

hard

A. [2, 4, 10, 8, 6]

B. [2, 4, 20, 8, 12]

C. [2, 4, 20, 8, 6]

D. [2, 4, 10, 8, 12]

Solution

1. Step 1: Double the value at index 2

   Value at index 2 is 6. Doubling it: 6 * 2 = 12. Update arr[2] = 12.

2. Step 2: Swap values at index 2 and index 4

   Value at index 4 is 10. Swap arr[2] and arr[4]: arr[2] = 10, arr[4] = 12.

3. Step 3: Write final array

   Array after operations: [2, 4, 10, 8, 12].

4. Final Answer:

   [2, 4, 10, 8, 12] -> Option B

5. Quick Check:

   Double then swap results in [2,4,10,8,12] [OK]

Hint: Double first, then swap values at given indexes [OK]

Common Mistakes:

• Swapping before doubling
• Mixing up indexes
• Not updating array correctly