Could it wait until a directory contains files i.e. not empty?

I want a build command to only run until a subdirectory contains files. Is it possible with `wait-on`?
Here is what worked:
```
"scripts" : {
"start:clipper": "wait-on popup/build/js && npm run watch_clipper"
}
```
Hower, this didn't work:
```
"scripts" : {
"start:clipper": "wait-on popup/build/* && npm run watch_clipper"
}
```
Even though the first one works, it's a little inflexible because the content of `popup/build` may change and no longer contains a `js` subdirectory.