Creating and indexing arrays

Arrays are JavaScript's lists

Real data comes in bunches: search results, chat messages, rows of a spreadsheet. An array lets one variable hold the whole bunch in order, which is what gives the loops from unit 3 something worth looping over.

What Python calls a list, JavaScript calls an array. Same square brackets, same zero-based indexing:

const fruits = ["apple", "banana", "cherry"];
console.log(fruits[0]);      // "apple"
console.log(fruits.length);  // 3

Two differences from Python to absorb:

• Length is the .length property (no parentheses), not len(...).
• Negative indexes do not work. fruits[-1] is undefined, not the last item. Use fruits[fruits.length - 1] or the modern method fruits.at(-1).

And one JavaScript quirk: reading an index that does not exist gives undefined instead of raising an error like Python's IndexError. Your program keeps running, often with confusing results later, so watch your bounds.