How to Remove the Promise <Hoge> Hoge

Please let me know if there is a website that explains how to use Promise <>

The detailed usage of MDN has been introduced in Japanese, so I think this will be helpful.
https://developer.mozilla.org/ja/docs/Web/JavaScript/Guide/Using_promises

I'm having trouble with navigator.mediaDevices.getUserMedia

I recognized that I wanted to take out the MediaStream that can be retrieved by getUserMedia and use it as a different function.
Would it be possible to solve this problem by calling another function from then?

constrender=(stream)=>{
  const video= document.getElementById("video");
  video.srcObject=stream;
  video.play();
};

navigator.mediaDevices.getUserMedia({video:true, audio:true})
.then(stream=>{
  render(stream);
})
.catch(error=>{
  console.error(error);
});