That's not really a huge improvement over just making a sparse array though, is it? Looks more confusing to me. Could be wrong but I'm pretty sure e.g. Array(1000000000) does not actually allocate much.
Or do you create an array of JSX-elements, like
let elements=[]
for (let i=0; i < 10; i++) {
elements.push(<li key={i}>I'm number {i}</li>)
}
return <ol>{elements}</ol>;