Think of it like you're just calling JavaScript functions. You can't use a for loop where the arguments to a function call would go:

return tbody(
    for (let i = 0; i < numrows; i++) {
        ObjectRow()
    } 
)

See how the function tbody is being passed a for loop as an argument – leading to a syntax error.

But you can make an array, and then pass that in as an argument:

const rows = [];
for (let i = 0; i < numrows; i++) {
    rows.push(ObjectRow());
}
return tbody(rows);

You can basically use the same structure when working with JSX:

const rows = [];
for (let i = 0; i < numrows; i++) {
    // note: we are adding a key prop here to allow react to uniquely identify each
    // element in this array. see: https://reactjs.org/docs/lists-and-keys.html
    rows.push(<ObjectRow key={i} />);
}
return <tbody>{rows}</tbody>;

Incidentally, my JavaScript example is almost exactly what that example of JSX transforms into. Play around with Babel REPL to get a feel for how JSX works.

Hi

When interpolating JSX, you need to either return JSX, or have the expression evaluate to an array where each element of the array is JSX.

For a for loop to work, you'd have to push to an array, then return that array at the end.

If you want to do it all inside the interpolated JSX:

return (
    <>
        {(() => {
            const arr = [];
            for (let i = 0; i < count; i++) {
                arr.push(
                    <div>
                        <h1>{i}</h1>
                    </div>
                );
            }
            return arr;
        })()}
    </>
)

But a more readable approach is to return the array itself, without putting it inside a fragment:

const count = 10;
const arr = [];
for (let i = 0; i < count; i++) {
    arr.push(
        <div>
            <h1>{i}</h1>
        </div>
    );
}
return arr;

But a for loop for this sort of situation is weird. The functional Array.from is better.

return Array.from(
    { length: 10 },
    (_, i) => (
        <div>
            <h1>{i}</h1>
        </div>
    )
);