Ordered Collections: Arrays

In JavaScript, Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Unlike arrays in many other languages, JavaScript arrays are Dynamic, meaning they can store mixed types and grow/shrink automatically in memory.

1. Array Basics and Indexing

Arrays use zero-based indexing.

const colors = ["red", "green", "blue"];
console.log(colors[0]); // "red"
console.log(colors.length); // 3

Splicing and Slicing:
- slice(start, end): Returns a new array containing a portion of the original (Non-destructive).
- splice(index, count, items...): Directly modifies the array by adding/removing elements (Destructive).

2. High-Order Array Methods (Functional)

Modern JavaScript development relies heavily on functional methods that treat arrays as immutable data sources.

map(): Creates a new array with the results of calling a provided function on every element.
filter(): Creates a new array with all elements that pass the test implemented by the provided function.
reduce(): Executes a "reducer" function on each element, resulting in a single output value (e.g., a sum or an object).

const numbers = [1, 2, 3, 4];
const sum = numbers.reduce((acc, curr) => acc + curr, 0); // 10

3. Searching and Sorting

find(): Returns the first element that satisfies a condition.
includes(): Checks if an array contains a specific value (returns Boolean).
sort(): Sorts elements in-place. Warning: By default, it converts elements to strings before sorting, which leads to [1, 10, 2] instead of [1, 2, 10]. Always provide a compare function: (a, b) => a - b.

4. Performance: Time Complexity

Understanding the "Cost" of array operations is vital for writing scalable code.

Operation	Method	Big O
Push / Pop	`push()`, `pop()`	O(1)
Shift / Unshift	`shift()`, `unshift()`	O(n) (Requires re-indexing)
Access	`arr[i]`	O(1)
Search	`find()`, `indexOf()`	O(n)

Interview Pro-Tips: The Array Spread Operator

Introduced in ES6, the spread operator [...] is the industry-standard way to copy or merge arrays without mutating the original.

const combined = [...arr1, ...arr2];

Technical Summary

Heterogeneity: Arrays can store different types, but for performance reasons (V8 optimizations), it is best to keep them homogeneous.
Mutability: Methods like push, pop, and splice modify the array in memory. Functional methods like map and filter return new arrays.
Stack vs Queue: Arrays can be used as Stacks (push/pop) or Queues (push/shift).