React Lists and Keys

const items = [1, 2, 3];
{items.map(item => (
    <li key={item}>{item}</li> # render each item
))}

<li key={item.id}>...</li> # use unique ID
<li key={index}>...</li> # use index (avoid if order changes)

const users = [{ id: 1, name: "John" }];
{users.map(user => (
    <div key={user.id}>
        {user.name}
    </div>
))}

const active = items.filter(item => item.active);
{active.map(item => (
    <li key={item.id}>{item.name}</li>
))}