There is actually a way to do exactly what the OP asked for. Just render and call the anonymous function like this:

render () {
  return (
    

      {(() => {
        if (someCase) {
          return (
            
someCase
          )
        } else if (otherCase) {
          return (
            
otherCase
          )
        } else {
          return (
            
catch all
          )
        }
      })()}
    
  )
}

Not exactly the same, but there is a workaround. There is a section about conditional rendering in the React documentation that you should take a look at. The following is an example of what you can do using inline if-else.

render() {
  const isLoggedIn = this.state.isLoggedIn;
  return (
    

      {isLoggedIn ? (
        
      ) : (
        
      )}
    
  );
}

You can also handle it inside the render function, but before returning jsx.

if (isLoggedIn) {
  button = ;
} else {
  button = ;
}

return (
  

    
    {button}
  
);

Also worth mentioning is what ZekeDroid brought up in the comments. If you are just checking a condition and don't want to render a specific piece of code that doesn't meet the condition, you can use the && operator.

return (
    

      
Hello!
      {unreadMessages.length > 0 &&
        

          You have {unreadMessages.length} unread messages.
        
      }
    
  );