JavaScript Array Cheat Sheet

Master JavaScript arrays with this comprehensive cheat sheet. Learn array methods, syntax, and best practices for working with arrays in JavaScript.

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Learn JavaScript: Free JavaScript Course & Array Cheat Sheet

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JavaScript Arrays: The Basics

An array is a fundamental data structure in JavaScript that stores multiple values in an ordered list. Learning about arrays is crucial for web development, as they help manage and manipulate data efficiently.

Declaring Arrays in JavaScript

const numbers = [1, 2, 3, 4, 5];

const fruits = ['apple', 'banana', 'cherry'];

Common JavaScript Array Methods

JavaScript provides various built-in methods for manipulating arrays. Mastering these will help you in JavaScript programming classes and real-world projects.

Adding & Removing Elements

• push() – Adds elements to the end.
• pop() – Removes the last element.
• unshift() – Adds elements to the beginning.
• shift() – Removes the first element.

const arr = [1, 2, 3];

arr.push(4); // [1, 2, 3, 4]

arr.pop(); // [1, 2, 3]

arr.unshift(0); // [0, 1, 2, 3]

arr.shift(); // [1, 2, 3]

Iterating Over Arrays

• forEach() – Loops through each item.
• map() – Creates a new array by applying a function to each element. Read more about map.
• filter() – Returns a new array with elements that meet a condition. Read more about filter.
• reduce() – Reduces an array to a single value. Read more about reduce.

const numbers = [1, 2, 3, 4, 5];

numbers.forEach(num => console.log(num));

const doubled = numbers.map(num => num * 2);

const evens = numbers.filter(num => num % 2 === 0);

const sum = numbers.reduce((acc, num) => acc + num, 0);

Searching & Transforming

• find() – Finds the first matching element.
• some() – Checks if at least one element meets a condition.
• every() – Checks if all elements meet a condition.
• indexOf() – Finds the index of an element.
• includes() – Checks if an element exists.

const nums = [10, 20, 30, 40];

console.log(nums.find(n => n > 25)); // 30

console.log(nums.includes(20)); // true

console.log(nums.indexOf(30)); // 2

Sorting & Reversing

• sort() – Sorts an array (modifies the original array).
• reverse() – Reverses the array order.
• toSorted() (ES2023) – Returns a sorted copy without modifying the original.

const scores = [40, 10, 100, 50];

scores.sort((a, b) => a - b); // [10, 40, 50, 100]

Removing Duplicates

const uniqueNumbers = [...new Set([1, 2, 2, 3, 4, 4, 5])]; // [1, 2, 3, 4, 5]

JavaScript Performance Tips

Optimizing your JavaScript code can improve efficiency, especially in web applications and server-side development.

1. Use push() instead of unshift() – push() is faster since it avoids shifting indexes.

2. Use Set for unique values – More efficient than filtering duplicates manually.

3. Use map() and reduce() wisely – map() creates a new array, while forEach() modifies in place.

4. Sort with compare functions – Default sort() converts values to strings, causing incorrect ordering.

JavaScript Array Interview Questions

If you're taking a JavaScript programming course or preparing for an interview, practice these questions:

1. Reverse an array in place – without using .reverse().
2. Find the missing number in a sequence.
3. Implement a custom map function that mimics Array.prototype.map.
4. Rotate an array by k positions.
5. Find the most frequent element in an array.

Additional JavaScript Resources

Looking for the best sites to learn JavaScript or JavaScript resources? Check these out:

• Array.some()
• Array.map()
• Array.filter()
• Array.reduce()

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