Arrays are a contiguous block of memory. That one property gives you O(1) random access by index — the most powerful primitive in algorithm design.

Almost every interview problem reduces to: traverse an array cleverly.

Key properties to memorise:

Indexing — where beginners lose points

Indices start at 0: the first element is arr[0], the last is arr[arr.length - 1]. The most common beginner bug in all of programming is the off-by-one:

for (let i = 0; i <= arr.length; i++)   // ❌ last pass reads arr[arr.length] — undefined!
for (let i = 0; i < arr.length; i++)    // ✓ visits exactly indices 0 … length-1

When in doubt, ask: how many times does this loop body run? For an array of length 3 the answer must be exactly 3 — i takes the values 0, 1, 2.

When an array is sorted, you unlock O(log n) search (binary search) and the two-pointer pattern — both are upcoming concepts.