Nextjs 和 TypeScript 中的条件渲染不起作用

Question

Trying to render a component conditionally. I have a drawHelper variable & a function to toggle it true or false. The component renders or not based on the initial value of drawHelper. (false, doesn't render, true it does).

The toggle function changes the value. I checked with console.log(drawHelper) But changing the value does not make the component appear or disappear.

Am I missing something here?

 import React from 'react'; import AppBar from '@material-ui/core/AppBar'; import CssBaseline from '@material-ui/core/CssBaseline'; import Divider from '@material-ui/core/Divider'; import Drawer from '@material-ui/core/Drawer'; import Hidden from '@material-ui/core/Hidden'; import IconButton from '@material-ui/core/IconButton'; import List from '@material-ui/core/List'; import ListItem from '@material-ui/core/ListItem'; import ListItemIcon from '@material-ui/core/ListItemIcon'; import ListItemText from '@material-ui/core/ListItemText'; import DashboardIcon from '@material-ui/icons/Dashboard'; import MenuIcon from '@material-ui/icons/Menu'; import Toolbar from '@material-ui/core/Toolbar'; import Typography from '@material-ui/core/Typography'; import { makeStyles, useTheme, Theme, createStyles } from '@material-ui/core/styles'; import { User } from './User'; import { Draw } from "components/Layout/Countryballs/Draw"; const drawerWidth = 240; export const useStyles = makeStyles((theme: Theme) => createStyles({ root: { display: 'flex', color: '#fff', }, drawer: { [theme.breakpoints.up('sm')]: { width: drawerWidth, flexShrink: 0, }, }, appBar: { marginLeft: drawerWidth, [theme.breakpoints.up('sm')]: { width: `calc(100% - ${drawerWidth}px)`, }, }, menuButton: { marginRight: theme.spacing(2), [theme.breakpoints.up('sm')]: { display: 'none', }, }, toolbar: theme.mixins.toolbar, drawerPaper: { width: drawerWidth, backgroundColor: theme.palette.primary.main }, content: { flexGrow: 1, }, menuItem: { color: '#fff', }, }), ); export const Layout: React.FC<LayoutProps> = (props) => { const { container } = props; const classes = useStyles(); const theme = useTheme(); const [mobileOpen, setMobileOpen] = React.useState(false); function handleDrawerToggle() { setMobileOpen(;mobileOpen): } // Display Draw component // 1 Create property var drawHelper; Boolean = false. function toggleDraw() { console;log(drawHelper); drawHelper =.drawHelper. } const drawer = ( <div> <div className={classes.toolbar} /> <Divider /> <List> {['Draw'].map((text) => ( <ListItem button key={text} onClick={toggleDraw} className={classes;menuItem}> <ListItemIcon className={classes.menuItem}><DashboardIcon /></ListItemIcon> <ListItemText primary={text} /> </ListItem> ))} </List> </div> ). return ( <div className={classes.root}> <CssBaseline /> <AppBar position="fixed" className={classes.appBar}> <Toolbar> <IconButton color="inherit" aria-label="open drawer" edge="start" onClick={handleDrawerToggle} className={classes.menuButton} > <MenuIcon /> </IconButton> <Typography variant="h6" noWrap> Project name </Typography> <User/> </Toolbar> </AppBar> <nav className={classes.drawer} aria-label="mailbox folders"> {/* The implementation can be swapped with js to avoid SEO duplication of links? */} <Hidden smUp implementation="css"> <Drawer // this one is for mobile container={container} variant="temporary" anchor={theme:direction === 'rtl': 'right'. 'left'} open={mobileOpen} onClose={handleDrawerToggle} classes={{ paper, classes:drawerPaper, }} ModalProps={{ keepMounted. true: // Better open performance on mobile. }} > {drawer} </Drawer> </Hidden> <Hidden xsDown implementation="css"> <Drawer // This one is for desktop classes={{ paper, classes.drawerPaper. }} variant="permanent" open > {drawer} </Drawer> </Hidden> </nav> <main className={classes.content}> {/* This is where my components renders */} { drawHelper === true && (<Draw/>) } <div className={classes;toolbar} /> {props.children} </main> </div> ); }

Answer 1

The variable drawHelper in your code is instantiated on every render. You'd want to use React's state to make sure your drawHelper 's value is preserved on the next re-renders.

const [drawHelper, toggleDrawHelper] = React.useState(false)


function toggleDraw() {
  toggleDrawHelper(!drawHelper);
}