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Version: 2020fa

Lecture 6

Lecture Slides

Lecture Video

Assignment 5 due 11/04 3:59pm

Conditional Rendering

Sometimes we only want things to render when a certain condition is met. For example, only display text when we meet a certain condition. React has conditional rendering to make this very simple.

PrelimTime.tsx
import React from 'react';

const PrelimTime = ({ prelimToday }: { readonly prelimToday: boolean }) => {
  if (prelimToday) {
    return <p>I have a prelim today.</p>;
  } else {
    return <p>I don't have a prelim today.</p>;
  }
};

export default PrelimTime;

In this example, we have a functional component PrelimTime that takes in a prop prelimToday. prelimToday is a boolean holding whether we have a prelim today or not. We want the component to display "I have a prelim today." if prelimToday is true and "I don't have a prelim today." if it is false.

Traditionally, we would use the if statement for this behavior (as shown above). We can also use conditional rendering to make writing this functionality more convenient.

First we can use the ternary operator:

PrelimTime.tsx
import React from 'react';

const PrelimTime = ({ prelimToday }: { readonly prelimToday: boolean }) => (
  prelimToday
    ? <p>I have a prelim today.</p>
    : <p>I don't have a prelim today.</p>;
);

export default PrelimTime;

The ternary operator is also very common in other languages as well such as Java or Python. The basic syntax is as follows:

[boolean expression] ? [true_result] : [false_result]

Before the ? you have your expression producing true or false. The part after the ? but before the : is the result/functionality you want if the boolean expression evaluates to true. The part after the : is what you want to happen if the expression is false.

Connecting with the PrelimTime example, my boolean expression was just the prop prelimToday, although in your code it can be a more complex boolean expression. If prelimToday is true, I display "I have a prelim today." If prelimToday is false, I display "I have a prelim today."

Notice though, how the only thing changing in this text is adding the word "don't" if prelimToday is false. So only if prelimToday is false, we want to add don't.

React supports the use of && operator:

PrelimTime.tsx
import React from 'react';

const PrelimTime = ({ prelimToday }: { readonly prelimToday: boolean }) => (
  <p>I {!prelimToday && "don't"} have a prelim today.</p>
);

export default PrelimTime;

Here, we display the text "I have a prelim today.". However, in the curly braces, if prelimToday is false then the word "don't" will be rendered. Conditional rendering with && is useful when you only have expected behavior for one branch of the conditional. In this case, I only had desired behavior if prelimToday was false.

As you have seen, React's conditional rendering made modifying render behavior based on conditions a lot easier. In this small example, we went from five lines of code in the component to just one!

Composition vs. Inheritance

Composition and inheritance are two programming techniques for defining how classes relate to objects. (Think of classes as the blueprint for a house and objects the actual houses created from that blueprint)

Composition

Composition defines a class as the sum of its individual parts. This is a "has-a" relationship (e.g. a car has a steering wheel, has a window, etc). In Java (and other object oriented languages), these components are represented as instance variables.

Inheritance

Inheritance derives one class from another. If class A is the parent of class B and C, B and C inherit the properties/functions of A. This is a "is-a" relationship (e.g. car is a vehicle, circle is a shape.)

React uses Composition

“React has a powerful composition model, and we recommend using composition instead of inheritance to reuse code between components.” -- React Docs

Containment

Components may not know their children ahead of time.

Children are the components you put within another component:

<ComponentA>{/* anything here is a child of Component A */}</ComponentA>

Use the children prop to pass in children components.

Container.tsx
import React, { ReactNode } from 'react';
type Props = { readonly children: ReactNode };
const Container = (props: Props) => (
  <div className="Border">{props.children}</div>
);
const App = () => (
  <div className="App">
    <Container>
      <p>Hello!</p>
      <p>Bye!</p>
    </Container>
  </div>
);

props.children will have the paragraph elements.

We didn't actually get to this live demo, adapted from this tutorial in the React docs, during lecture but it is very simple if you want to try it out yourself. We also show how to import styles.

Container.tsx
import React, { ReactNode } from 'react';
import './Container.css'; // this is how we import styles

type Props = { readonly children: ReactNode };

const Container = (props: Props) => (
  <div className="Border">{props.children}</div>
);

export default Container;
Container.css
.Border {
  border: 4px solid black;
  background-color: azure;
}

Less common but you also may want multiple "holes" in your component (for example, a left and right child):

SplitPane.tsx
import React, { ReactNode } from 'react';
import './SplitPane.css';

type Props = { readonly left: ReactNode; readonly right: ReactNode };

const SplitPane = (props: Props) => (
  <div>
    <div className="LeftPane">{props.left}</div>
    <div className="RightPane">{props.right}</div>
  </div>
);

export default SplitPane;
SplitPane.css
/* these colors are ugly I know */
.LeftPane {
  float: left;
  width: 50%;
  background-color: red;
}

.RightPane {
  float: right;
  width: 50%;
  background-color: aquamarine;
}
App.jsx
import React from 'react';
import SplitPane from './SplitPane';
import Container from './Container';

const App = () => {
  return (
    <div className="App">
      <Container>
        <p>Hello, world!</p>
      </Container>
      <SplitPane
        left={<div>I'm on the left!</div>}
        right={<div>I'm on the right!</div>}
      />
    </div>
  );
};

export default App;

Lifting State Up

This section was a live demo, adapted from this tutorial in the React docs.

App.tsx
import React, { useState } from 'react';
import './App.css';
import FahrenheitInput from './FahrenheitInput';
import CelsiusInput from './CelsiusInput';

function App() {
  const [temperature, setTemperature] = useState(-40);

  return (
    <div className="App">
      <label>Fahrenheit:</label>
      <FahrenheitInput
        temperature={temperature}
        callback={(temp) => setTemperature(temp)}
      />
      <br />
      <label>Celcius:</label>
      <CelsiusInput
        temperature={temperature}
        callback={(temp) => setTemperature(temp)}
      />
      <br />
      {temperature >= 100 ? (
        <span>Water would boil here!</span>
      ) : (
        <span>Water would not boil here!</span>
      )}
      <br />
      <span>Water would {temperature >= 0 && 'not'} freeze here!</span>
    </div>
  );
}

export default App;
CelsiusInput.tsx
import React, { ChangeEvent } from 'react';

type Props = {
  readonly temperature: number;
  readonly callback: (temperature: number) => void;
};

const CelsiusInput = ({ temperature, callback }: Props) => {
  const handlechange = (e: ChangeEvent<HTMLInputElement>) =>
    callback(parseInt(e.target.value) || 0);

  return <input value={temperature} onChange={handlechange} />;
};

export default CelsiusInput;
FahrenheitInput.tsx
import React from 'react';

type Props = {
  readonly temperature: number;
  readonly callback: (temperature: number) => void;
};

const FahrenheitInput = ({ temperature, callback }: Props) => {
  const handlechange = (e: ChangeEvent<HTMLInputElement>) =>
    callback((((parseInt(e.target.value) || 0) - 32) * 5) / 9);

  return <input value={(temperature * 9) / 5 + 32} onChange={handlechange} />;
};

export default FahrenheitInput;

Check lecture video for a more detailed explanation.