I am trying to render a component if (age<18) in the next block of code
var split_dob = dateOfBirth.split("-");
var month = split_dob[1];
var day = split_dob[2];
var year = split_dob[0];
var dob_asdate = new Date(year, month, day);
var today = new Date();
var mili_dif = Math.abs(today.getTime() - dob_asdate.getTime());
var age = (mili_dif / (1000 * 3600 * 24 * 365.25));
console.log(age);
if(age<18){setEligible(true)}
here's useState hook:
const [uneligible,setEligible] = React.useState(
false // default value
)
and here's how I am trying to render it:
<div>{uneligible&& <Alert variant="filled" severity="error">
This is an error alert — check it out!
</Alert>}</div>
in the actual project there's no errors, just the component not rendering and I made minimised version of the code here but it still has problem in rendering: https://codesandbox.io/s/thirsty-sara-jiomm?file=/src/App.js
You don't actually need state here. Just use const uneligible = age<18
and get rid of the useState
code, and it will work okay. If you put uneligible
in the state you create a render loop, as setting it triggers a re-render, which sets the state again and triggers another re-render, etc.
const ProfilePage = (props) => {
var dateOfBirth = "2007-01-01";
var split_dob = dateOfBirth.split("-");
var month = split_dob[1];
var day = split_dob[2];
var year = split_dob[0];
var dob_asdate = new Date(year, month, day);
var today = new Date();
var mili_dif = Math.abs(today.getTime() - dob_asdate.getTime());
var age = mili_dif / (1000 * 3600 * 24 * 365.25);
console.log(age);
const uneligible = age < 18;
return (
<div>
{uneligible && (
<Alert variant="filled" severity="error">
This is an error alert — check it out!
</Alert>
)}
</div>
);
};
Get the dateOfBirth
, and set uneligible
as a standard const ( useMemo
for slight optimization if needed). This will prevent the loop in which render sets the state, and setting the state causes a re-render, which sets the state...
const { useMemo } = React; const ProfilePage = ({ dateOfBirth }) => { const uneligible = useMemo(() => { const mili_dif = Math.abs(Date.now() - Date.parse(dateOfBirth)); const age = mili_dif / (1000 * 3600 * 24 * 365.25); return age < 18; }, [dateOfBirth]); return ( <div> {uneligible && ( <div variant="filled" severity="error"> This is an error alert — check it out; </div> )} </div> ); }. ReactDOM,render( <ProfilePage dateOfBirth="2007-01-01" />; root );
<script crossorigin src="https://unpkg.com/react@17/umd/react.development.js"></script> <script crossorigin src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script> <div id="root"></div>
The function re-renders again and again because you set the state immediately. Solution:
import React from "react";
import Alert from "@material-ui/lab/Alert";
const ProfilePage = (props) => {
const isEligible = () => {
var dateOfBirth = "2007-01-01";
var split_dob = dateOfBirth.split("-");
var month = split_dob[1];
var day = split_dob[2];
var year = split_dob[0];
var dob_asdate = new Date(year, month, day);
var today = new Date();
var mili_dif = Math.abs(today.getTime() - dob_asdate.getTime());
var age = mili_dif / (1000 * 3600 * 24 * 365.25);
console.log(age);
return age < 18;
}
return (
<div>
{isEligible() && (
<Alert variant="filled" severity="error">
This is an error alert — check it out!
</Alert>
)}
</div>
);
};
export default ProfilePage;
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