What is the purpose of sx prop in Material UI?

Question

<Box style={{ padding: "20px" }}>
  <Post post={post} setCurrentId={setCurrentId} />
</Box>

<Box sx={{ padding: "20px" }}>
  <Post post={post} setCurrentId={setCurrentId} />
</Box>

React.js + Material UI

The above two examples do the same thing, one uses sx prop in material UI, the other is regular inline styling using css, so what is the point of "sx"? and should it always be used over style={{}} when using Material UI?

Answer 1

The sx prop is a shortcut for defining custom style that has access to the theme

It can accept any CSS properties plus a few extra from MUI.

There are differences like :

1. shortHand: padding-top can be written as pt.
2. access to theme: If you define your theme in material UI, sx prop can directly access its properties like color. Example from the doc :

import * as React from 'react';
import { Box, ThemeProvider, createTheme } from '@mui/system';

const theme = createTheme({
  palette: {
    background: {
      paper: '#fff',
    },
    text: {
      primary: '#173A5E',
      secondary: '#46505A',
    },
    action: {
      active: '#001E3C',
    },
    success: {
      dark: '#009688',
    },
  },
});

export default function Example() {
  return (
    <ThemeProvider theme={theme}>
      <Box
        sx={{
          bgcolor: 'background.paper',
          boxShadow: 1,
          borderRadius: 2,
          p: 2,
          minWidth: 300,
        }}
      >
        <Box sx={{ color: 'text.secondary' }}>Sessions</Box>
        <Box sx={{ color: 'text.primary', fontSize: 34, fontWeight: 'medium' }}>
          98.3 K
        </Box>
        <Box
          sx={{
            color: 'success.dark',
            display: 'inline',
            fontWeight: 'bold',
            mx: 0.5,
            fontSize: 14,
          }}
        >
          +18.77%
        </Box>
        <Box sx={{ color: 'text.secondary', display: 'inline', fontSize: 14 }}>
          vs. last week
        </Box>
      </Box>
    </ThemeProvider>
  );
}

Properties in sx object can have functions as valuse. These functions have access to theme too.

<Box sx={{ height: (theme) => theme.spacing(10) }} />

3. Grid properties: gap, rowGap and columnGap are available in sx.
4. Responsive styles: You can define properties according to the different MUI device size shorthands:

borderColor : { xs: "red", sm: "green" },

5. Access to child components: You can change styles of chlid components using nested styles:

<TextField
  variant="outlined"
  sx={{
    '& .MuiInputBase-input': {
      color: 'white',
    },
  }}
/>

6. Much more than CSS: The sx prop also supports CSS syntax including child and pseudo-selectors, media queries etc. Eg:

<Box
  sx={{
    // some styles
    ":hover": {  //psuedo-selectors
      boxShadow: 6,
    },
    '@media print': {  //media-queries
      width: 300,
    },
  }}
>

Sources:

1.

2.