Common array operations in Javascript

Introduction

Arrays help us store lists of things. Common operations let us add, remove, or find items easily.

You want to keep a list of your favorite movies.

You need to add new tasks to a to-do list.

You want to check if a name is in a list of friends.

You want to remove finished items from a shopping list.

You want to count how many times a word appears in a list.

Syntax

Javascript

class SimpleArray {
  constructor() {
    this.items = [];
  }

  add(item) {
    this.items.push(item);
  }

  remove(item) {
    const index = this.items.indexOf(item);
    if (index !== -1) {
      this.items.splice(index, 1);
    }
  }

  contains(item) {
    return this.items.includes(item);
  }

  size() {
    return this.items.length;
  }

  print() {
    console.log(this.items);
  }
}

The push method adds an item to the end of the array.

The splice method removes items by index.

Examples

Adds one item to an empty array and prints it.

Javascript

const myArray = new SimpleArray();
myArray.add('apple');
myArray.print();

Trying to remove an item from an empty array does nothing.

Javascript

const myArray = new SimpleArray();
myArray.remove('apple');
myArray.print();

Removes the first item when there are multiple items.

Javascript

const myArray = new SimpleArray();
myArray.add('apple');
myArray.add('banana');
myArray.remove('apple');
myArray.print();

Checks if items are in the array.

Javascript

const myArray = new SimpleArray();
myArray.add('apple');
console.log(myArray.contains('apple'));
console.log(myArray.contains('banana'));

Sample Program

This program shows how to add, check, and remove items from an array wrapped in a class. It prints the array before and after changes.

Javascript

class SimpleArray {
  constructor() {
    this.items = [];
  }

  add(item) {
    this.items.push(item);
  }

  remove(item) {
    const index = this.items.indexOf(item);
    if (index !== -1) {
      this.items.splice(index, 1);
    }
  }

  contains(item) {
    return this.items.includes(item);
  }

  size() {
    return this.items.length;
  }

  print() {
    console.log(this.items);
  }
}

const fruits = new SimpleArray();
console.log('Initial array:');
fruits.print();

fruits.add('apple');
fruits.add('banana');
fruits.add('cherry');
console.log('After adding items:');
fruits.print();

console.log('Contains banana?', fruits.contains('banana'));
console.log('Contains grape?', fruits.contains('grape'));

fruits.remove('banana');
console.log('After removing banana:');
fruits.print();

fruits.remove('grape');
console.log('After trying to remove grape (not in list):');
fruits.print();

Important Notes

Adding an item with push is fast, O(1) time.

Removing an item requires finding it first, so it takes O(n) time where n is the array size.

Common mistake: Trying to remove an item not in the array does nothing, so always check if it exists if needed.

Use contains to check presence before removing if you want to avoid silent failures.

Summary

Arrays store lists of items in order.

Use push to add items at the end.

Use indexOf and splice to remove items by value.

Use includes to check if an item is in the array.

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