Why is this blog by Firebase team using array-contains while the guide by Firebase team using array_contains?
What is the difference between array-contains and array_contains?
It's the same problem with Angularfire2.
It's array-contains, as shown in the JavaScript code samples.
Since the text is the same for all languages, it might not always use the operators/functions of the language that you're looking for. But I'm pretty sure it's supposed to use JavaScript syntax, so I'm not sure why the documentation text is different here. I filed a bug for it.