The reason has to do with JSX being turned into regular JavaScript without knowing what code will be hit or not. When the Babel transpiler runs, it turns your first version of your code into this:
var Square = React.createClass({
displayName: "Square",
render: function render() {
return React.createElement("div", null);
}
});
When you don’t specify anything to return, you get an error because the render method must call React.createElement. Not having anything there causes an error like Unexpected Token or something equivalent.
