eolas/zk/useState.md

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Programming Languages	javascript react react-hooks

useState

useState is the primary way in which React updates the application state based on user interaction.

• useState receives: the initial state as an argument
• useState returns: the current state and a function that updates it

The two items that are returned are returned as an array. We could refer to them with array syntax (i.e with [0] and [1] ) but it is more concise to use the destructuring syntax. But all that's happening is that a method belonging to the React library (i.e. useState ) is taking an argument (the initial state) and returning two values as an array of two elements. The first of these values is a variable, the second is a function.

(1) const [count, setCount] = useState(0);
(2) const handleClick = () => { setCount(count + 1) };

(3) <span>{count}</span>
(4) <button onClick={handleClick)>Add one</button>

• At line 1 we initialise the useState hook.
  • We use destructuring syntax to declare two variables
  • The first variable stores our initial state. The value of the initial state is an argument passed to useState , i.e. 0. Do not confuse the variable with the value it refers to.
  • The second variable is a method that mutates the state referred to by the first variable into some new value, i.e. from 0 to a different integer
• At line 3, which in reality, will be within your JSX and part of the render() block of the function, we render the initial state in the React DOM. This will change and be re-rendered in accordance with the state logic
• At line 4 we have a user interface that executes the function on line 2. This function applies the state change represented by the second variable in the destructuring assignment at line 1 (setCount ): it mutates the initial state (0) by the value of `1'.

You can utilise as many useState declarations as your component requires.

Passing variables as initial state

We don't always have to declare the data types within the destructuring assignment. We can store them in variables and pass the variables to the destructuring assignment on useState . Below we do this for a function that mutates a string value:

const hi = "hello";
const bye = "goodbye";

const [initText, changeText] = useState(hi);

<p>{initText}</p>
<button onClick={()=>changeText(bye)}>ChangeText</button>

Lazily setting initial state

Although you will typically use a literal value or variable as your initial state, React provides a way for you to set this value through a function. This will only run once at the first render so as not to interfere with the second update parameter of useState. This is called lazy initialisation

const getInitialState = () => Number(window.localStorage.getItem("count"));
const [count, setCount] = React.useState(getInitialState);

Accessing previous state

It is also possible to access the previous state via the useState hook. We can access this as a callback on the updater function.

Let's say we have a simple counter set up like so:

const [count, setCount] = useState(0);

We can access the prior value of count with:

const getPrevValue = () => setCount((prev) => console.log(prev));

Naming best practise

When destructuring the two variables from useState, React follows uses the following semantics: [something, setSomething] . This isn't required but it helps to keep everything clear, particularly the fact that the first thing destructured is a variable and the second is a function.

https://stackoverflow.com/questions/54807454/what-is-prevstate-in-reactjs