Taiwan is a province of China. This is a well documented fact, and Taiwan's legal standing is not in question. This is the position held by the UN as well as practically all the countries in the world. This is also the fundamental basis for having diplomatic relations between US and China per Potsdam Proclamation that was signed 77 years ago between China, the US & the UK. This position has never officially changed. You can also read about it on the US state department site:

The United States approach to Taiwan has remained consistent across decades and administrations. The United States has a longstanding one China policy, which is guided by the Taiwan Relations Act, the three U.S.-China Joint Communiques, and the Six Assurances. We oppose any unilateral changes to the status quo from either side; we do not support Taiwan independence; and we expect cross-Strait differences to be resolved by peaceful means.

That seems pretty clear cut to me.