Curly Brackets in Arrow Functions

The pair of braces forms a block, containing a list of statements. You need to use a return statement explicitly to make the function return something:

(one) => {
    return oneTodo(one, action);
//  ^^^^^^
}

If you omit the braces, the arrow function has a concise body, which consists solely of a single expression whose result will implicitly become the return value of the function.

case 'toggleTodo' :
    return (
        state.map( (one) => 
            oneTodo( one, action )
        )
    );

is equal to:

case 'toggleTodo' :
    return (
        state.map( (one) => {
            return oneTodo( one, action )
        })
    );

see the return statement