React fundamentals


React is used to describe the UI.

ReactDOM is used to use the description to render it.

To render UI with React, we need:

  • a React element (described with JSX)
  • a dom element (get by document.getElementById or document.querySelector)

Let's get rid of JSX

  • change JSX to React.createElement

  • React is just JavaScript, because it's just function call

  • that means you can pass any JS variable into React.createElement, and doing weird things like passing a IIFE into React.createElement

  • since you can be as weird as you want, it means you can be as abstract as you want.

  • The signature of React.createElement:

    • after first 2 parameter, all the remaining parameters will be the children of the element.
    • other than string, the children of React.createElement can also be another react element.
    • the second parameter is called props, which you can treat it as HTML attributes
    • most props is just like HTML attributes, with few edge cases. E.g. for style, instead of passing a string, react expects an object. The reason for that is object is easier to compose and remove.
  • Let's go back from React.createElement back to JSX:

    • Note that with JSX, {} means go back to JavaScript. Without that everything in it will be treated as string.
    • as such, the style props will have the double curly {{}}. Note that there is no special meaning of double curly in React. The first {} mean go back to JavaScript, while the second one is the object we pass to style props.
  • Let's import FaHome from react-icons/fa and render it. FaHome is a react component. A react components wrapped some React elements that we can use. Let's see how we can create a react component like FaHome.

  • To define a react component, just create a function that will returns some react elements. Then you can use it just like HTML element like div or span. With that we can render similar elements multiple times.

  • But we're not satisfied with render the component multiple times. Rendering a react component with entirely is not what we want. We want to make react component customizable.

    • Let's think about the native button element, we can pass classname to it to make it style differentl. Same goes for React component, we should be able to customize our component by passing different props.
    • The way to do that is the react component (which is a function), accepts a parameter (which is usually called props). We can returns different react elements based on the props we receives in the component.
  • Making component more flexible.

    • Our current abstraction (icon and text props) is not a very flexible abstraction because it dictates it must have an icon and the text. We want to allow others to pass any thing as they wish and this component only responsible to render the button and style it accordingly like this:
    <FaPlus />
    • To allow that, we can use a special props in React: children.

      • children is anything that you pass within the open and closed tag of a JSX.
      • React will combine multiple elements into single children props.
      • Technically you can still pass children by name, like <PrimaryButton children="Add">. Conceptually what React do is to move the elements from within the tags into this children props.

Data can flows from parent to children with props. But how do we pass the data from children to parent? For example, how can we allows PrimaryButton to inform the parent that it has been clicked?

  • To allows that to happen, we pass function as props. Then the children component can call the function.
const PrimaryButton = (props) => (
<button onClick={() => props.onClick()}>{props.children}</button>
<PrimaryButton onClick={() => console.log('button has been clicked')}>
<FaPlus />
  • Some people call those props with function as value as "callback props", some call it "event props", but fundamentally there are just another props. React call everything as props because function is just another type of variable and React accept any type of variable as props.

  • Note that if we don't pass the props into the underlying button component, React will not magically recognize that it should pass down the onClick props. It's just JavaScript at the end of the day.

We can create not just DOM event like onClick, we can also create custom event like onKeyboardClick, or even <CustomerForm onSuccess={(customer) => {}}>, which give us a higher level of abstraction to understand our application.

Let's do some code-along with some libraries to get a taste of how it feels.


React is a declarative library.

When we say declarative, it means we just need to talk about "what", instead of "how".

The metaphor of "what" and "how" is the difference of the old and new Myvi.

In my old Myvi, I need to control the fan and the coldness of the air. If the outer temperature changes, I need to tweaks those two control to make myself comfortable. But what I actually want is just to maintain temperature at the level I want.

In the new Myvi, you just need to set a temperature, and the cooling system will just auto-adjust the temperature accordingly. There is no more tweaking, you just say, I want "21 C", then boom, it will figure it out itself.

To use the state hooks in React, this is how you do it:

import * as React from 'react';
const Component = () => {
const [value, setValue] = React.useState(5)


The line can be actually broken down to:

const stateArray = React.useState(5); // 5 is the initial value
const value = stateArray[0]; // first item of the array is the value
const setValue = stateArray[1]; // second item of the array is the value setter

You can have multiple states by calling useState multiple times.

Generally, in React we like to think in states. Instead of thinking "what to do" when something happens, in React most of the time what you do is just update the states.

And your UI will automatically updated based on states.

But how does it work?

Let's inspect the output of our JSX by console.log it.

You will see a tree data structure.

What React does under the hood is rerun your component to generate a new set of tree and comparing them to figure out required changes.

const element = Component();
// states get updated
const newElement = Component();
const operations = findDiff(element, newElement);

Let's try to think how UI development is without React.

Instead of deriving a UI from the state, let's try to update it our selve by updating it manually.

const element = document.getElementById('some-id');
element.innerText = message.length;
<span id="some-id" />

But in React we usually don't hardcode id in our component, because it is invalid if there are multiple instance of the components.

Instead, we use a ref.

const msgLengthRef = React.useRef(null);
msgLengthRef.current.innerText = message.length;
<span ref={msgLengthRef}>

There is a problem with this imperative approach. If message is updated via other mean, the length will be incorrect.

So we have to remember to update the length in those cases too.

The root cause of the issue is we are very used to think in terms of events: when something happens, do something.

In React, what you should think should be instead of states: when something happens, state must be updated.

Therefore, thinking in events is kinda tricky because you need to figure all the events that may cause the message to changed, including initial page load.

When we thinking of states, we can describe what we want as: "whenever the message changes, the length display should be updated as well".

To achieve that in React, we can use another hook in React: the effect hook.

React.useEffect(() => {
msgLengthRef.current.innerText = message.length;
}, [message]);

Effect hook allows you to have granular control on when something should happens, e.g.

const firstTwenty = message.substring(0, 20);
React.useEffect(() => {
document.title = firstTwenty ? `New Post: ${firstTwenty}` : 'Site title';
}, [firstTwenty]);

Let's do some exercise that showcase:

  • how the lint rule help you to write effect hooks correctly
  • how the lint rule tell you cannot use hooks conditionally

The most common use case of a frontend application is making API calls, and that's what we gonna do here.

To make a API calls, usually we need a state hook and another effect hook:

const [value, setValue] = React.useState(undefined);
React.useEffect(() => {
apiCalls(id).then((res) => setValue(res));

But we will have an infinite loop, to fix that we need pass the id to the effect hooks dependencies:

const [value, setValue] = React.useState(undefined);
React.useEffect(() => {
apiCalls(id).then((res) => setValue(res));
}, [id]);

We still have another problem here, but this time it's more subtle.

If we click around we may get a React error about unmount component cannot setState. To fix that, we need to use the cleanup function of effect hooks.

The effect hooks signature actually looks like:

React.useEffect(() => {
// operation depending on dependencies
return functionThatCleanupOperation;
}, [...dependencies]);

For the api calls, the simplest way to do that is to add a local variable isCurrent:

const [value, setValue] = React.useState(undefined);
React.useEffect(() => {
let isCurrent = true;
apiCalls(id).then((res) => {
if (isCurrent) {
return () => {
isCurrent = false;
}, [id]);