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>
</>
);
}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])
}previous state in useEffect
function usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
});
return ref.current;
}
const Component = (props) => {
const {receiveAmount, sendAmount } = props
const prevAmount = usePrevious({receiveAmount, sendAmount});
useEffect(() => {
if(prevAmount.receiveAmount !== receiveAmount) {
// process here
}
if(prevAmount.sendAmount !== sendAmount) {
// process here
}
}, [receiveAmount, sendAmount])
}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);usestate access previous state
const [arrayOfObjs, handleObjSelection] = useState([]);
// on a buttton for example
<button
onClick={selectedObj => handleObjSelection(
prevSelected => [...prevSelected, selectedObj],
))}
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