prevstate in usestate
const [prevState, setState] = React.useState([]);
setState(prevState => [...prevState, 'somedata'] );prevstate in usestate
const [prevState, setState] = React.useState([]);
setState(prevState => [...prevState, 'somedata'] );useRef
/*
A common use case is to access a child imperatively:
*/
function TextInputWithFocusButton() {
const inputEl = useRef(null);
const onButtonClick = () => {
// `current` points to the mounted text input element
inputEl.current.focus();
};
return (
<>
<input ref={inputEl} type="text" />
<button onClick={onButtonClick}>Focus the input</button>
</>
);
}usestate previous state
const [stateObject, setObjectState] = useState({
firstKey: '',
secondKey: '',
});
setObjectState((prevState) => ({
...prevState,
secondKey: 'value',
}));
// stateObject = {
// firstKey: '',
// secondKey: 'value',
// }useeffect previous state
const Component = (props) => {
const {receiveAmount, sendAmount } = props
// declare usePrevious hook
const usePrevious = (value) => {
const ref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
}
// call usePrevious hook on component state variables to store previousState
const prevAmount = usePrevious({receiveAmount, sendAmount});
// useEffect hook to detect change of state variables
useEffect(() => {
if(prevAmount.receiveAmount !== receiveAmount) {
// process here
}
if(prevAmount.sendAmount !== sendAmount) {
// process here
}
}, [receiveAmount, sendAmount])
}useReducer
function init(initialCount) { return {count: initialCount};}
function reducer(state, action) {
switch (action.type) {
case 'increment':
return {count: state.count + 1};
case 'decrement':
return {count: state.count - 1};
case 'reset': return init(action.payload); default:
throw new Error();
}
}
function Counter({initialCount}) {
const [state, dispatch] = useReducer(reducer, initialCount, init); return (
<>
Count: {state.count}
<button
onClick={() => dispatch({type: 'reset', payload: initialCount})}> Reset
</button>
<button onClick={() => dispatch({type: 'decrement'})}>-</button>
<button onClick={() => dispatch({type: 'increment'})}>+</button>
</>
);
}when to use previous state in useState
import React, { useState } from "react";
import ReactDOM from "react-dom";
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<h1>{count}</h1>
<button onClick={() => setTimeout(() => setCount(count + 1), 2000)}>
Delayed Counter (basic)
</button>
<button onClick={() => setTimeout(() => setCount(x => x + 1), 2000)}>
Delayed Counter (functional)
</button>
<button onClick={() => setCount(count + 1)}>Immediate Counter</button>
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<Counter />, rootElement);Copyright © 2021 Codeinu
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