add useWidth to ts demos

docs/src/pages/components/use-media-query/UseWidth.tsx

@@ -0,0 +1,38 @@
+import React from 'react';
+import { Theme, ThemeProvider, useTheme, createMuiTheme } from '@material-ui/core/styles';
+import useMediaQuery from '@material-ui/core/useMediaQuery';
+import { Breakpoint } from '@material-ui/core/styles/createBreakpoints';
+
+type BreakpointOrNull = Breakpoint | null;
+
+/**
+ * Be careful using this hook. It only works because the number of
+ * breakpoints in theme is static. It will break once you change the number of
+ * breakpoints. See https://reactjs.org/docs/hooks-rules.html#only-call-hooks-at-the-top-level
+ */
+function useWidth() {
+  const theme: Theme = useTheme();
+  const keys: Breakpoint[] = [...theme.breakpoints.keys].reverse();
+  return (
+    keys.reduce((output: BreakpointOrNull, key: Breakpoint) => {
+      // eslint-disable-next-line react-hooks/rules-of-hooks
+      const matches = useMediaQuery(theme.breakpoints.up(key));
+      return !output && matches ? key : output;
+    }, null) || 'xs'
+  );
+}
+
+function MyComponent() {
+  const width = useWidth();
+  return <span>{`width: ${width}`}</span>;
+}
+
+const theme = createMuiTheme();
+
+export default function UseWidth() {
+  return (
+    <ThemeProvider theme={theme}>
+      <MyComponent />
+    </ThemeProvider>
+  );
+}