React - my learning journey
In June of 2018 I started learning and working with React. In this blog post I will share some things I learned in the beginning and came across or find worth noting.
I would be very happy if other beginners can learn something or find a place to start their React learning journey right here.
- How does React work?
- What is JSX?
- React Components
- What are Props?
- What is State?
- How does conditional rendering work in React?
- How are events handled in React?
- Controlled Components vs Uncontrolled Components
- What are default props?
- What are PropTypes?
- React Context
- What are children?
- What are higher-order-components
- What is Redux?
With React you can build interactive user interfaces, which get easily updated when the data changes. Dealing with data changes and DOM manipulation can be slow and unnecessary.
In React for every DOM object there is a virtual DOM object (like a lightweight copy). So React creates a virtual DOM. When state changes in a component it firstly runs a "diffing" algorithm, which identifies what has changed in the virtual DOM. The second step is reconcilitation, where it updates the DOM with the results of diff.
- When a JSX element is rendered, every virtual DOM element is updated.
- React then compares the virtual DOM with a snapshot that was taken right before the update.
- By doing this, React figures out exactly which virtual DOM objects have changed (diffing)
- Then React updates only thos real DOM objects that have changed.
- saved in a variable
- passed into a function
- stored in an array or an object
They can have attributes like HTML elements, they can be nested (if they take up more than one line they must be wrapped in paranthesis) andthey must have at least one outermost element. In JSX HTML class get replaced by className. JS in JSX in JS is possible by wrapping content in
A React component is a small, resuable chunk of code, responsible for one job (e.g. render some HTML). The name of a component hast to start with a capital letter.
Components render other components (like a page renders a nav bar). To use a component from another file you need to import the file. You can also pass information to a component.
Props (short for properties) are a way of passing data/information from the parent to the child. Props are for static data and you can access the props with this.props. It is possible to declare default Props (for the case that no props are available).
A Component can not change their props. Props do not have to just be data. Callback functions my be passed in as props.
State is data which is changing over time. State is reserved for interactivity and is private and fully controlled by a component. State can be updated asynchronously. A parent component can pass its state down to a child as props. As soon as the state changes a rerender is triggered.
To handle events in React, you have to add an event listener to an element. (difference to HTML: must be in lowerCamelCase e.g. onClick instead of onclick)
Event listeners are provided when the element is initially rendered, so there is no need to call
addEventListener. The event listener gets passed in an event handler as props. Naming convention: start with handle, e.g. handleClick.
An input element whos value is controlled by React is called a controlled component. In HTML forms maintain their own state and update it based on user input. In React, mutable state is kept in the state property of the component.
These two combined make React the "single source of truth". Then the React component that renders a form also controls what happens in that form on subsequent user input, so you write an event handler for every state update.
The alternative to controlled components are uncontrolled components, where form data is handled by the DOM itself.
It is possible to define default values for component props. They are defined as a property on the component.
This is useful when you don't get certain data, which you normally pass in props. Then you can fallback to the props, which you defined as the default.
With PropTypes you can check the type of a prop on a component. This means you can say you only accept a prop, when it has the type of a string. You can also flag props as required and set defaults as described above.
Context provides a way to share data between components without having to pass props down manually at every level. This means, Context lets you pass a value deep into the component tree without explicitly threading it through every component.
But keep in mind, that Context should not be used only to avoid props drilling. The best use case for it is when a lot of components need to access the same prop.
Context ist defined with
React.createContext. You have to use a Provider to pass the current context to the tree below. Then you have to use a Consumer to read the current context, React will find the closest Provider and use its value.
You can use children on components that represent generic boxes and don't know their children ahead of time. It is used to display whatever you include between the opening and closing tags when invoking a component.
A higher-order component is a function that takes a component and returns a new component. It is also a wrapper for the old component. It is not part of the React API per se.
Redux is an advanced topic and not part of React. It is an additional library which comes in handy for problems, React does not solve properly for some developers.
Redux give you a global state container, the store. So you have one global state for your whole application and every component can access and update it by using actions and reducers.
You can trigger an action inside of your component. The reducer always listen for specific actions and then talk to the store and return the updated state.
Sources & Further Reading
React has one of the best documentations I have ever used. I regularly head over to there, to fresh up my knowledge. It is written very well and has a lot of useful examples in it.
This blog post was originally published on August 15, 2019.
I wish you a wonderful day! Marco
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