Trump pardoned Steve Bannon for example and he has not yet been convicted (he has been charged). Assange has been charged with various things by the Feds, which is what Trump would pardon. The floated theory went that Trump could plausibly pardon himself as well, for any likely future charges.
It's not questionable if you can pardon things that haven't been charged yet. You can. The pardon power is absolute, greater than laws that define specific crimes, and doesn't require accepting guilt or whatever.
For a specific example, Jimmy Carter pardoned everyone who dodged the draft for Vietnam[1]. This showed that pardons could both apply to actions which hadn't been charged and categories of people.
I think one of the only limits on the pardon power is that it can only apply to past behavior.
[1] https://www.justice.gov/pardon/vietnam-war-era-pardon-instru...
It doesn't actually show that because the legal effect of thar pardon has never been challenged. If no one tries to charge someone subject to a pardon, or that pardon recipient never offers the pardon against the charges, nothing is resolved about the validity and effect of the pardon.
That's because impeachment is a political process, not a judicial proceeding. Clemency does not affect impeachment.
The pardon power applies to offenses against the US “except in cases of impeachment” (Art. II § 2 ¶ 1). This uncontroversially means a pardon cannot affect the process of impeachment in the House and trial on charges of impeachment in the Senate. It is disputed whether it also somehow prevents pardons for criminal charges relating to the same act for which the recipient was impeached. The issue has never come up in a nontheoretical sense, so an authoritative resolution hasn’t been handed down.
Ford pardoning Nixon is the usual example.