Common array operations in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the push() method do in an array?

It adds one or more elements to the end of an array and returns the new length of the array.

beginner

How do you remove the first element from an array?

Use the shift() method. It removes the first element and returns it, shifting all other elements down by one index.

intermediate

What is the difference between map() and forEach()?

map() creates a new array by applying a function to each element, while forEach() simply runs a function on each element without returning a new array.

beginner

How can you find the index of a specific element in an array?

Use the indexOf() method. It returns the first index at which the element can be found, or -1 if it is not present.

intermediate

What does the filter() method do?

It creates a new array with all elements that pass a test implemented by a provided function.

Which method adds elements to the end of an array?

A pop()

B unshift()

C shift()

D push()

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

What does the pop() method do?

A Removes the first element

B Removes the last element

C Adds an element at the start

D Adds an element at the end

pop() removes the last element from an array and returns it.

Which method creates a new array by applying a function to each element?

A map()

B reduce()

C filter()

D forEach()

map() applies a function to each element and returns a new array.

How do you find the position of an element in an array?

A filter()

B find()

C indexOf()

D slice()

indexOf() returns the index of the first occurrence of an element.

Which method returns a new array with elements that pass a test?

A filter()

B map()

C reduce()

D forEach()

filter() returns a new array with elements that pass the test function.

Explain how to add and remove elements from the start and end of an array in JavaScript.

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

Describe the difference between map(), forEach(), and filter() methods.

Focus on what each method returns and how they process array elements.

Practice

1. Which method adds a new item to the end of a JavaScript array?

easy

A. shift()

B. pop()

C. push()

D. unshift()

Solution

1. Step 1: Understand array methods for adding/removing items

   push() adds items to the end, pop() removes from the end, shift() removes from the start, unshift() adds to the start.

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

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

3. Final Answer:

   push() -> Option C

4. Quick Check:

   push() adds item at end [OK]

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

Common Mistakes:

• Confusing push() with pop()
• Using shift() to add items
• Using unshift() to add at end

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

easy

A. arr.shift()

B. arr.unshift()

C. arr.push()

D. arr.pop()

Solution

1. Step 1: Recall array methods for removing items

   pop() removes last item, shift() removes first item, push() adds last, unshift() adds first.

2. Step 2: Choose method that removes first item

   shift() removes the first item from the array.

3. Final Answer:

   arr.shift() -> Option A

4. Quick Check:

   shift() removes first item [OK]

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

Common Mistakes:

• Using pop() to remove first item
• Using push() or unshift() to remove items
• Confusing shift() with unshift()

3. What is the output of the following code?

const fruits = ['apple', 'banana', 'cherry'];
const index = fruits.indexOf('banana');
if (index !== -1) {
  fruits.splice(index, 1);
}
console.log(fruits);

medium

A. ['apple', 'banana', 'cherry']

B. ['banana', 'cherry']

C. ['apple']

D. ['apple', 'cherry']

Solution

1. Step 1: Find the index of 'banana' in the array

   indexOf('banana') returns 1 because 'banana' is at position 1.

2. Step 2: Remove 'banana' using splice

   splice(1, 1) removes 1 item at index 1, so 'banana' is removed.

3. Final Answer:

   ['apple', 'cherry'] -> Option D

4. Quick Check:

   Removed 'banana' from array [OK]

Hint: indexOf finds position, splice removes item at that position [OK]

Common Mistakes:

• Not checking index before splice
• Using splice with wrong index
• Expecting splice to return new array

4. The following code is intended to remove 'orange' from the array, but it doesn't work as expected. What is the error?

const colors = ['red', 'green', 'blue'];
const index = colors.indexOf('orange');
colors.splice(index, 1);
console.log(colors);

medium

A. indexOf returns -1 if item not found, causing splice to remove last item

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

C. indexOf should be replaced with includes()

D. splice() cannot remove items by index

Solution

1. Step 1: Check indexOf result for 'orange'

   Since 'orange' is not in the array, indexOf returns -1.

2. Step 2: Understand splice behavior with negative index

   splice(-1, 1) removes the last item, so 'blue' is removed unexpectedly.

3. Final Answer:

   indexOf returns -1 if item not found, causing splice to remove last item -> Option A

4. Quick Check:

   Check index before splice to avoid removing wrong item [OK]

Hint: Check indexOf result before splice to avoid errors [OK]

Common Mistakes:

• Not checking if index is -1
• Using includes() instead of indexOf for index
• Assuming splice can't remove by index

5. Given an array nums = [1, 2, 3, 4, 5], which code snippet correctly removes all even numbers from the array?

hard

A. nums = nums.filter(n => n % 2 === 0);

B. nums = nums.filter(n => n % 2 !== 0);

C. nums.splice(1, 2);

D. nums.pop(); nums.pop();

Solution

1. Step 1: Understand filtering even numbers

   To remove even numbers, keep only numbers where n % 2 !== 0 (odd numbers).

2. Step 2: Check each option

   nums.filter(n => n % 2 !== 0) keeps odd numbers, removing all evens. nums.filter(n => n % 2 === 0) keeps evens. nums.splice(1, 2) removes items at fixed indexes. nums.pop() twice removes only the last two items.

3. Final Answer:

   nums = nums.filter(n => n % 2 !== 0); -> Option B

4. Quick Check:

   filter odd numbers to remove even [OK]

Hint: Use filter with condition to keep desired items [OK]

Common Mistakes:

• Using filter condition backwards
• Using splice without index knowledge
• Using pop to remove specific items