If-Else in JSX

if-else statements don't work inside JSX. This is because JSX is just syntactic sugar for function calls and object construction. Take this basic example:

// This JSX:
ReactDOM.render(<div id="msg">Hello World!</div>, mountNode);

// Is transformed to this JS:
ReactDOM.render(React.createElement("div", {id:"msg"}, "Hello World!"), mountNode);

This means that if statements don't fit in. Take this example:

// This JSX:
<div id={if (condition) { 'msg' }}>Hello World!</div>

// Is transformed to this JS:
React.createElement("div", {id: if (condition) { 'msg' }}, "Hello World!");

That's not valid JS. You probably want to make use of a ternary expression:

ReactDOM.render(<div id={condition ? 'msg' : null}>Hello World!</div>, mountNode);

If a ternary expression isn't robust enough, you can use if statements outside of your JSX to determine which components should be used:

var loginButton;
if (loggedIn) {
  loginButton = <LogoutButton />;
} else {
  loginButton = <LoginButton />;
}

return (
  <nav>
    <Home />
    {loginButton}
  </nav>
);

Or if you prefer a more "inline" aesthetic, define immediately-invoked function expressions inside your JSX:

return (
  <section>
    <h1>Color</h1>
    <h3>Name</h3>
    <p>{this.state.color || "white"}</p>
    <h3>Hex</h3>
    <p>
      {(() => {
        switch (this.state.color) {
          case "red":   return "#FF0000";
          case "green": return "#00FF00";
          case "blue":  return "#0000FF";
          default:      return "#FFFFFF";
        }
      })()}
    </p>
  </section>
);

Note:

In the example above, an ES6 arrow function is utilized to lexically bind the value of this.

Try using it today with the Babel REPL.