Arrays & Zero-Based Indexing

Variables: const, let & var (The Box Metaphor)

Learn how variables act as labeled containers for data using const, let, and var to manage lexical scope and reassignment safely in modern applications.

1. Ordered Lists of Data

An array is an ordered list of values enclosed in square brackets [...]. Array elements can hold any data type, numbers, strings, objects, or even nested arrays.

2. Why It Matters

Arrays store sequences of data (user lists, product catalogs, order items) accessible by 0-based numerical index positions.

3. Zero-Based Indexing & Length

Array positions start at index 0 (first item = arr[0]). You can find the last item using arr[arr.length - 1] or modern arr.at(-1):

const fruits = ["Apple", "Banana", "Cherry"];

console.log(fruits[0]);      // "Apple"  (First element)
console.log(fruits[1]);      // "Banana" (Second element)
console.log(fruits.length);  // 3
console.log(fruits.at(-1));  // "Cherry" (Last element)

// Modifying array contents
fruits.push("Dragonfruit"); // Add item to end
console.log(fruits.length); // 4

4. Core Array Mutating Methods

• push(item): Add to end of array.
• pop(): Remove item from end of array.
• unshift(item): Add item to start of array.
• shift(): Remove item from start of array.

const stack = [10, 20];
stack.push(30);  // [10, 20, 30]
stack.pop();     // Returns 30 -> [10, 20]

5. Common Mistakes

• Out-of-Bounds Access: Reading an index that does not exist (like fruits[99]) returns undefined rather than throwing an error. Always check .length or verify array bounds.
• Confusing Array Length with Last Index: An array with length = 5 has valid indices from 0 to 4. Accessing arr[5] returns undefined!

6. Next Steps

Arrays store ordered lists. Next, let's explore Objects for key-value records!