How to run eslint in Terminal / Shell

npm install && npm i -g eslint && npx eslint --ext .tsx --ext .ts ./

Please heed that npm install is used here to start my React application, which uses it as a build tool. IMportant

npm i -g eslint will install eslint, if it does not exist yet (for example a CI with Jenkins).

The npx eventually executes eslint for all files with ending .tsx or .ts recursively in the current directory. Please adapt the endings according to your needs – when you do not use TypeScript but JavaScript change it to .js

React Hook Component Conditional Rendering

The HTML is rerendered after click and displays the chosen content.

We use a stateful variable.

The stateful variable (here, a boolean) is called ‚condition‘ 😉 :

const [condition, setCondition] = useState(false);

The #useEffect method to react on variable state change:

useEffect(() => {
    ...
},[condition]);

The methods for the buttons:

const handleClickSomething01 = () => setCondition(true);
const handleClickSomething02 = () => setCondition(false);

The returned HTML:

if (condition === true) {
    return (
        <div>
            <Button variant="contained" 
                    color="primary" 
                    onClick={handleClickSomething02}>
                Primary
            </Button>
            <Button variant="contained" 
                    color="secondary" 
                    onClick={handleClickSomething01}>
                Secondary
            </Button>CONDITION fullfilled
        </div>
    );
} else {
    return (
        <div>
            <Button variant="contained" 
                    color="primary" 
                    onClick={handleClickSomething02}>
                Primary
            </Button>
            <Button variant="contained" 
                    color="secondary" 
                    onClick={handleClickSomething01}>
                Secondary
            </Button>CONDITION not fulfilled
        </div>
    );
}

You will notice that this example uses duplicated code (sort of, a few details differ).
Why did I do this?
I have debugged so many unmaintainable Frontends during the last two decades and nearly always the obstacles where too many detailed conditions in the HTML.

Instead, by using IF/ELSE and clearly depict the HTML without conditional renderings within, the Code will be easier to maintain in the long run.

Of course, this is my personal experience and also a matter of Clean Code paradigms.
In some cases, am using inline conditions, inside the HTML lines, too. But as we Architects like to say: IT DEPENDS 😉




React – use CSS depending on states (change class according to a variable’s state)

For this example I use React Hooks and a Component with a stateful variable.
When the variable is changed, the CSS class is changed accordingly.
The part in the component is automatically re-rendered on the fly.

//The variable is called 'active'. 
//This is an example. Choose any name.
const [active, setActive] = useState(0);

We use the constant #getStyle to return a CSS class name based on the variable.

    const getStyle = (input) => {
        if (input === active){
            // This example is freely chosen. 
            //button-active is the CSS class' name
            return "button-active";
        } else {
            return "button-inactive";
        }
    };

The return statement returns rendered HTML.

//The hard coded value 0 can be any dynamic 
//variable from JS or React, like one from the 
//props (Properties) object
return (
  <div className={getStyle(0)}>
  </div>
);
  • Stateful variable called ‚active‘
  • Const named #getStyle with an input parameter
  • render method returning HTML with a css class name
  • Input is provided to the #getStyle const
  • If the variable ‚active‘ is changed by the application somewhere, the HTML is re-rendered
  • The input is compared to the variable and the name of the CSS class returned

React – stateful variables and objects (State) & Hooks & Components

Imagine you want to re-render a Component when a variable changes its state.

Your Component:

function MyComponent(props) {

Your variable, a name:

const [name, setName] = useState("OLD NAME");

A #useEffect method to react to changes of that specific variable:

useEffect(()=>{
    alert('new name!');//a small alert to show you the effect immediately
    setName('NEW NAME');
}, [name]);

…and a return with that name being rendered:

return (
    <div className={"someFancyCssClass"}>{name}</div>            
);

Note that, in this example, the initial OLD NAME is immediately replaced.
Reason for it: we set the name during initialization in the const.

React – Howto redirect to other component

Sometimes, for example on a login, you want to redirect the User to another page in your application.

We assume that your Application has the default address of http://localhost:3000.

In this example, the application has the path localhost:3000 and you can navigate to the news with /news.

import {Redirect} from "react-router-dom";
//... your program code goes here...

return(
   <Redirect to='news'/>
);

The return statement will redirect to localhost:3000/news

React Hook Lifecycle – control WHEN functions are executed

By defining the dependencies in your #useEffect method, you can control the execution of methods and the rendering.

Execute only once when component is first rendered
By passing an empty array for a #useEffect method, you guarantee that the method is only executed once:

useEffect(() => {
     //do something....
}, [])

Now it behaves like #componentDidMount in the old React class architecture.

Execute after every render cycle
If no array is passed to #useEffect, the method will be executed AFTER EVERY render cycle:

useEffect(() => {
     //do something...
})//no array here

Execute only if some value has changed
If #useEffect shall only be executed when a certain variable has changed, do:

useEffect(() => {
     //do something...
   ...myValue...
}, [myValue])//myValue defined in the array