Modifying arrays in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add an element to the end of an array in JavaScript?

Use the push() method. For example, arr.push(element) adds element to the end of arr.

beginner

What method removes the last element from an array?

The pop() method removes the last element and returns it. It changes the original array.

beginner

How can you add an element to the beginning of an array?

Use the unshift() method. For example, arr.unshift(element) adds element to the start of arr.

intermediate

What does the splice() method do?

It can add, remove, or replace elements in an array at any position. For example, arr.splice(start, deleteCount, item1, item2) removes deleteCount elements from start and inserts new items.

beginner

How do you change the value of an element at a specific index in an array?

Assign a new value directly using the index. For example, arr[2] = newValue changes the third element to newValue.

Which method adds an element to the end of an array?

A push()

B pop()

C shift()

D unshift()

The push() method adds an element to the end of an array.

What does arr.pop() do?

A Adds an element to the end

B Adds an element to the start

C Removes the first element

D Removes the last element

pop() removes and returns the last element of the array.

How do you remove elements from the middle of an array?

A Using <code>slice()</code>

B Using <code>splice()</code>

C Using <code>push()</code>

D Using <code>pop()</code>

splice() can remove elements from any position in the array.

Which method adds elements to the beginning of an array?

A unshift()

B push()

C pop()

D shift()

unshift() adds elements to the start of an array.

How do you change the value of the second element in an array named arr?

A arr.set(2, newValue)

B arr[2] = newValue

C arr[1] = newValue

D arr.change(1, newValue)

Array indexes start at 0, so the second element is at index 1.

Explain how to add and remove elements from both ends of an array in JavaScript.

Think about methods that work at the start and end of the array.

Describe how the splice() method can be used to modify an array.

Consider how you can remove and add items anywhere in the array.

Practice

1. Which method adds one or more elements to the end of a JavaScript array?

easy

A. push()

B. pop()

C. shift()

D. unshift()

Solution

1. Step 1: Understand array methods for adding elements

   push() adds elements to the end, unshift() adds to the start.

2. Step 2: Identify the method that adds to the end

   push() is the method that adds elements at the end of the array.

3. Final Answer:

   push() -> Option A

4. Quick Check:

   Adding at end = push() [OK]

Hint: push() adds at end, unshift() adds at start [OK]

Common Mistakes:

• Confusing push() with pop()
• Using shift() to add elements
• Thinking unshift() adds at end

2. Which of the following is the correct syntax to remove the first element from an array arr?

easy

A. arr.pop()

B. arr.shift()

C. arr.unshift()

D. arr.push()

Solution

1. Step 1: Recall methods that remove elements

   pop() removes from end, shift() removes from start.

2. Step 2: Identify method to remove first element

   shift() removes the first element of the array.

3. Final Answer:

   arr.shift() -> Option B

4. Quick Check:

   Remove first element = shift() [OK]

Hint: shift() removes first, pop() removes last [OK]

Common Mistakes:

• Using pop() to remove first element
• Using unshift() which adds elements
• Confusing push() with removal methods

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

let arr = [1, 2, 3, 4];
arr.splice(1, 2, 8, 9);
console.log(arr);

medium

A. [1, 8, 9, 4]

B. [1, 2, 3, 4]

C. [8, 9, 4]

D. [1, 8, 9]

Solution

1. Step 1: Understand splice parameters

   splice(1, 2, 8, 9) means start at index 1, remove 2 elements, then insert 8 and 9.

2. Step 2: Apply splice on array

   Original array: [1, 2, 3, 4]. Remove elements at index 1 and 2 (2 and 3), then insert 8 and 9 at index 1. Result: [1, 8, 9, 4].

3. Final Answer:

   [1, 8, 9, 4] -> Option A

4. Quick Check:

   splice removes and inserts = [1, 8, 9, 4] [OK]

Hint: splice(start, deleteCount, items...) edits array in place [OK]

Common Mistakes:

• Confusing splice with slice
• Not removing correct elements
• Forgetting splice inserts new items

4. The following code tries to remove the element '3' from the array but does not work as expected. What is the error?

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

medium

A. The index 3 is out of range for removal

B. The splice method is used incorrectly; it should be pop()

C. The index 3 removes the wrong element, should be 2

D. splice cannot remove elements from arrays

Solution

1. Step 1: Identify the element to remove and its index

   The element '3' is at index 2 in the array [1, 2, 3, 4].

2. Step 2: Check the splice call

   The code calls arr.splice(3, 1), which removes the element at index 3 (which is 4), not 3.

3. Final Answer:

   The index 3 removes the wrong element, should be 2 -> Option C

4. Quick Check:

   Remove '3' at index 2, not 3 [OK]

Hint: Check array indexes carefully before splice [OK]

Common Mistakes:

• Using wrong index for splice
• Confusing pop() with splice()
• Thinking splice can't remove elements

5. You have an array arr = [5, 10, 15, 20]. You want to replace the middle two elements with 12 and 18 using a single method call. Which code correctly does this?

hard

A. arr.splice(2, 2, 12, 18);

B. arr.splice(0, 2, 12, 18);

C. arr.splice(1, 1, 12, 18);

D. arr.splice(1, 2, 12, 18);

Solution

1. Step 1: Identify indexes of middle two elements

   Array is [5, 10, 15, 20]. Middle two elements are at indexes 1 and 2 (10 and 15).

2. Step 2: Use splice to replace these elements

   Call arr.splice(1, 2, 12, 18) to start at index 1, remove 2 elements, and insert 12 and 18.

3. Final Answer:

   arr.splice(1, 2, 12, 18); -> Option D

4. Quick Check:

   Replace middle two with splice(1,2,12,18) [OK]

Hint: splice(startIndex, countToRemove, ...newItems) replaces items [OK]

Common Mistakes:

• Wrong start index
• Removing too few or too many elements
• Using splice with wrong parameters