[Select] The value matches with reference equality, document it

[LikiaSun]

Select component's onChange is work, but it cannot render selected value.

import React, { useState } from 'react';
import InputLabel from '@material-ui/core/InputLabel';
import Select from '@material-ui/core/Select';
import MenuItem from '@material-ui/core/MenuItem';

export default function Test() {
  const [test, setTest] = useState({
    a: '',
    b: '',
    sum: {
      name: '',
    },
  });

  const option = [{ na: 'aa', nb: 'bb' }, { na: 'cc', nb: 'dd' }, { na: 'ee', nb: 'gg' }];

  const handleTestChange = e => {
    e.preventDefault();
    setTest({
      ...test,
      a: e.target.value.na,
      b: e.target.value.nb,
      sum: {
        ...test.sum,
        name: `${e.target.value.na} vs ${e.target.value.nb}`,
      },
    });
  };

  return (
    <div>
      <h1>test</h1>
      <h5>{test.sum.name}</h5>
      <InputLabel>Preview test</InputLabel>
      <Select value={test.sum.name} onChange={handleTestChange}>
        {option.map(option => (
          <MenuItem
            key={`${option.na + option.nb}`}
            value={{ na: option.na, nb: option.nb }}
          >{`${option.na} vs ${option.nb}`}</MenuItem>
        ))}
      </Select>
    </div>
  );
}

result like that

also develop tool didn't pop any error or warning

[eps1lon]

@LikiaSun Please provide a full reproduction test case. This would help a lot 👷 .
A live example would be perfect. This codesandbox.io template may be a good starting point. Thank you!

[LikiaSun]

hmm 🤔️ did i just write all of code on the top?
fine
LIVE SAMPLE: https://codesandbox.io/s/determined-satoshi-li3zb

[eps1lon]

hmm thinking did i just write all of code on the top?

You did but provided no environment which we asked for. You should've been presented with an issue template that includes which values are relevant. A codesandbox is an alternative to a completely filled out issue template.

[eps1lon]

If you use objects in value you need to make sure these pass referential equality check. So if you have an array of values e.g. [{id: 1}, {id: 2}] you can't use <Select value={{id: 1}}><MenuItem value={{id: 1}} /></Select> but something like <Select value={values[selectedIndex]}><MenuItem value={values[0]} ></Select>.

We should include this caveat in the docs.

[oliviertassinari]

We should include this caveat in the docs.

@eps1lon Agree, it's a common problem people face. With some digging, we can find duplicated issues in the repository.

[LikiaSun]

And also I just saw Select.renderValue api to let multple value or result value can be render to Select field.

https://codesandbox.io/s/serene-star-0bxs4

[prettyannoying]

Hi, is this issue resolved? Can I take it up, if not?

[oliviertassinari]

@skenypatel you are free to go :), regarding the solution, it seems that it's about mentionning the === equality check in the value prop description.

[oliviertassinari]

@LikiaSun I think that we should wait for the documentation to be updated before closing the issue :).

[prettyannoying]

@skenypatel you are free to go :), regarding the solution, it seems that it's about mentionning the === equality check in the value prop description.

@oliviertassinari I'm new to this, I did not understand what exactly I am supposed to do. Can anyone guide me?

[adeelibr]

@skenypatel You need to update the value prop documentation in filepackages\material-ui\src\Select\Select.js 😄

Particularly on Line no 190 as show below 😄

https://github.com/mui-org/material-ui/blob/0f28b62cc9d5b1c070f8959abb7eb9458e42c417/packages/material-ui/src/Select/Select.js#L190-L195