I find that when I define a state with the value '1',
and set the state with the value '2' every time when I click a button,
the first two times will cause re-render
reproduce demo: https://codesandbox.io/s/sweet-brattain-ys11d
code: using react@17 without strict mode
import { useState } from "react";
export default function App() {
const [a, setA] = useState("1");
console.log("render", a);
return (
<button
onClick={() => {
setA("2");
}}
>
{a}
</button>
);
}
// log:
// render 1
// render 2
// render 2
I can understand the first re-render because the state changed to '2' from '1',
but I don't understand the second re-render
I think this explains the anomaly very well:
If you update a State Hook to the same value as the current state, React will bail out without rendering the children or firing effects. (React uses the Object.is comparison algorithm.)
Note that React may still need to render that specific component again before bailing out. That shouldn't be a concern because React won't unnecessarily go “deeper” into the tree. If you're doing expensive calculations while rendering, you can optimize them with useMemo
Note the last paragraph. This is quoted directly from here .
I think something missing in the code. I checked on my end. it's working fine as per our expectations.
Thanks
import React from "react";
import "./styles.css";
import { useState } from "react";
export default function App() {
const [a, setA] = useState(1);
console.log("render", a);
return (
<button
onClick={() => {
setA(a + 1);
}}
>
{a}
</button>
);
}
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