A function that will be called with each directory until it returns a `string` with the path, which stops the search, or the root directory has been reached and nothing was found. Useful if you want to match files with certain patterns, set of permissions, or other advanced use-cases.
When using async mode, the `matcher` may optionally be an async or promise-returning function that returns the path.
Allow symbolic links to match if they point to the chosen path type.
### findUp.exists(path)
Returns a `Promise<boolean>` of whether the path exists.
### findUp.sync.exists(path)
Returns a `boolean` of whether the path exists.
#### path
Type: `string`
Path to a file or directory.
### findUp.stop
A [`Symbol`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol) that can be returned by a `matcher` function to stop the search and cause `findUp` to immediately return `undefined`. Useful as a performance optimization in case the current working directory is deeply nested in the filesystem.
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