Arrays

An array is an ordered, changeable collection — JavaScript's equivalent of Python's list, and probably the most-used data structure in the language.

let fruits = ["apple", "banana", "cherry"];
console.log(fruits);
console.log(fruits[0]);
console.log(fruits[fruits.length - 1]);

fruits.push("date");
console.log(fruits);

fruits.pop();
console.log(fruits);

console.log(fruits.length);

[ 'apple', 'banana', 'cherry' ]
apple
cherry
[ 'apple', 'banana', 'cherry', 'date' ]
[ 'apple', 'banana', 'cherry' ]
3

• fruits[0] gets the first item — unlike Python, JavaScript has no negative indexing, so fruits[fruits.length - 1] is how you get the last item
• push() adds an item to the end of the array, in place
• pop() removes and returns the last item
• length is a property that always reflects the array's current size

Sorting — and a real gotcha

let numbers = [5, 3, 1, 4, 2];
numbers.sort();
console.log(numbers);
numbers.reverse();
console.log(numbers);

[ 1, 2, 3, 4, 5 ]
[ 5, 4, 3, 2, 1 ]

That looks right — but sort() actually converts elements to strings and compares them alphabetically by default, which happens to give the correct order for single digits. It breaks as soon as a number has more than one digit:

let bigNumbers = [10, 1, 21, 2];
bigNumbers.sort();
console.log(bigNumbers);

[ 1, 10, 2, 21 ]

"10" sorts before "2" alphabetically, the same way "apple" sorts before "banana" — that's genuinely what happened here. To sort numbers correctly, pass a comparator function: numbers.sort((a, b) => a - b).

Next: objects — storing values by name instead of by position.