Assign a value to a variable in JSX for ReactJS

You can create a function outside the render() that will get correct value:

function getStory(storyObj) {
  return storyObj.type === 'story' && storyObj.story ? storyObj.story : storyObj;
}

And then use it in JSX:

{props.stories.map(storyObj =>
  <div key={storyObj.id}>
     <SliderItem story={getStory(storyObj)} />
  </div>
)}

Everything between {} in the JSX is just JavaScript, so you can do the following:

{props.stories.map((story) => {
  const storyObj = (story.type === 'story' && story.story) ? story.story : story;
  return (
    <div key={story.id}>
      <SliderItem story={storyObj} />
    </div>
  );
})}