A clear primer on JavaScript arrays: what they are, creating them with [], and accessing elements via zero-based indexing. It highlights pitfalls (out-of-range indexes, undefined values), shows a practical e-commerce example using reduce() to total cart prices, offers tips for handling missing data and writing clean code, and recommends books—emphasizing that mastering indexing leads to more efficient, reliable JavaScript.

Declaring a variable with `const` in JavaScript makes its value constant, but assigning an object or array only freezes the reference. Modifying properties or elements is allowed without error, while reassigning the entire object or array triggers a `TypeError`.

The `const` keyword in JavaScript prevents reassignment of variables but doesn't automatically freeze objects or arrays, allowing their properties and elements to be modified. To ensure immutability, use the `Object.freeze()` method to prevent changes.

Four essential JavaScript array methods every fullstack developer should know are `push()`, `pop()`, `shift()`, and `unshift()`, allowing you to add or remove elements from the beginning or end of an array, making it easier to manage data in web development.

In JavaScript, `const` prevents reassignment of a variable, not modification of its contents, especially with objects and arrays. To achieve true immutability, techniques like Object.freeze() or Immutable.js are needed.

The .length property in JavaScript returns the number of elements in an array, essential for looping, indexing, and using array methods. Access the .length property to get an array's length: `const colors = ['red', 'green', 'blue']; console.log(colors.length); // Output: 3`.