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An array is an ordered list of values. JavaScript arrays are dynamic — they grow and shrink as you add and remove elements. A single array can hold mixed types (though in practice, keep types consistent for clarity).

Creating arrays

Mutating methods (modify in place)

Non-mutating methods (return a new array)

The spread operator ... is the modern way to copy/concat. Mutating methods are footguns when you accidentally share references — non-mutating methods avoid that.

Searching

Iterating

forEach doesn't return anything useful. For transformations, use map (next lesson).

Length is mutable!

Assigning to length resizes the array. Setting it to 0 empties it.

Common mistakes

• Out-of-bounds access returns undefined silently — no exception. Always sanity-check.
• Comparing arrays with === — checks reference identity, not contents. [1] === [1] is false.
• Mutating sort() and reverse() when you wanted a new array. Use [...arr].sort() instead.
• Default sort() sorts as STRINGS: [10, 2, 1].sort() is [1, 10, 2]. Pass a comparator: .sort((a, b) => a - b).