conditional rendering is not working in my functional component

Question

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

Oblosys · Accepted Answer · 2021-01-23 22:56:17Z

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>
  );
};

Sandbox example

Ori Drori · Accepted Answer · 2021-01-23 22:53:15Z

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>

jzProg · Accepted Answer · 2021-01-23 22:56:32Z

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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