TranslateProject/sources/tech/20180201 Conditional Rendering in React using Ternaries and.md

Conditional Rendering in React using Ternaries and Logical AND

Photo by Brendan Church on Unsplash

There are several ways that your React component can decide what to render. You can use the traditional if statement or the switch statement. In this article, we’ll explore a few alternatives. But be warned that some come with their own gotchas, if you’re not careful.

Ternary vs if/else

Let’s say we have a component that is passed a name prop. If the string is non-empty, we display a greeting. Otherwise we tell the user they need to sign in.

Here’s a Stateless Function Component (SFC) that does just that.

const MyComponent = ({ name }) => {
  if (name) {
    return (
      <div className="hello">
        Hello {name}
      </div>
    );
  }
  return (
    <div className="hello">
      Please sign in
    </div>
  );
};

Pretty straightforward. But we can do better. Here’s the same component written using a conditional ternary operator.

const MyComponent = ({ name }) => (
  <div className="hello">
    {name ? `Hello ${name}` : 'Please sign in'}
  </div>
);

Notice how concise this code is compared to the example above.

A few things to note. Because we are using the single statement form of the arrow function, the return statement is implied. Also, using a ternary allowed us to DRY up the duplicate <div className="hello"> markup. 🎉

Ternary vs Logical AND

As you can see, ternaries are wonderful for if/else conditions. But what about simple if conditions?

Let’s look at another example. If isPro (a boolean) is true, we are to display a trophy emoji. We are also to render the number of stars (if not zero). We could go about it like this.

const MyComponent = ({ name, isPro, stars}) => (
  <div className="hello">
    <div>
      Hello {name}
      {isPro ? '🏆' : null}
    </div>
    {stars ? (
      <div>
        Stars:{'⭐️'.repeat(stars)}
      </div>
    ) : null}
  </div>
);

But notice the “else” conditions return null. This is becasue a ternary expects an else condition.

For simple if conditions, we could use something a little more fitting: the logical AND operator. Here’s the same code written using a logical AND.

const MyComponent = ({ name, isPro, stars}) => (
  <div className="hello">
    <div>
      Hello {name}
      {isPro && '🏆'}
    </div>
    {stars && (
      <div>
        Stars:{'⭐️'.repeat(stars)}
      </div>
    )}
  </div>
);

Not too different, but notice how we eliminated the : null (i.e. else condition) at the end of each ternary. Everything should render just like it did before.

Hey! What gives with John? There is a 0 when nothing should be rendered. That’s the gotcha that I was referring to above. Here’s why.

According to MDN, a Logical AND (i.e. &&):

expr1 && expr2

Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.

OK, before you start pulling your hair out, let me break it down for you.

In our case, expr1 is the variable stars, which has a value of 0. Because zero is falsey, 0 is returned and rendered. See, that wasn’t too bad.

I would write this simply.

If expr1 is falsey, returns expr1, else returns expr2.

So, when using a logical AND with non-boolean values, we must make the falsey value return something that React won’t render. Say, like a value of false.

There are a few ways that we can accomplish this. Let’s try this instead.

{!!stars && (
  <div>
    {'⭐️'.repeat(stars)}
  </div>
)}

Notice the double bang operator (i.e. !!) in front of stars. (Well, actually there is no “double bang operator”. We’re just using the bang operator twice.)

The first bang operator will coerce the value of stars into a boolean and then perform a NOT operation. If stars is 0, then !stars will produce true.

Then we perform a second NOT operation, so if stars is 0, !!stars would produce false. Exactly what we want.

If you’re not a fan of !!, you can also force a boolean like this (which I find a little wordy).

{Boolean(stars) && (

Or simply give a comparator that results in a boolean value (which some might say is even more semantic).

{stars > 0 && (

A word on strings

Empty string values suffer the same issue as numbers. But because a rendered empty string is invisible, it’s not a problem that you will likely have to deal with, or will even notice. However, if you are a perfectionist and don’t want an empty string on your DOM, you should take similar precautions as we did for numbers above.

Another solution

A possible solution, and one that scales to other variables in the future, would be to create a separate shouldRenderStars variable. Then you are dealing with boolean values in your logical AND.

const shouldRenderStars = stars > 0;

return (
  <div>
    {shouldRenderStars && (
      <div>
        {'⭐️'.repeat(stars)}
      </div>
    )}
  </div>
);

Then, if in the future, the business rule is that you also need to be logged in, own a dog, and drink light beer, you could change how shouldRenderStars is computed, and what is returned would remain unchanged. You could also place this logic elsewhere where it’s testable and keep the rendering explicit.

const shouldRenderStars = 
  stars > 0 && loggedIn && pet === 'dog' && beerPref === 'light`;

return (
  <div>
    {shouldRenderStars && (
      <div>
        {'⭐️'.repeat(stars)}
      </div>
    )}
  </div>
);

Conclusion

I’m of the opinion that you should make best use of the language. And for JavaScript, this means using conditional ternary operators for if/elseconditions and logical AND operators for simple if conditions.

While we could just retreat back to our safe comfy place where we use the ternary operator everywhere, you now possess the knowledge and power to go forth AND prosper.

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作者简介：

Managing Editor at the American Express Engineering Blog http://aexp.io and Director of Engineering @AmericanExpress. MyViews !== ThoseOfMyEmployer.

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via: https://medium.freecodecamp.org/conditional-rendering-in-react-using-ternaries-and-logical-and-7807f53b6935

作者：Donavon West 译者：译者ID 校对：校对者ID

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