import {
  copyN,
  StringReader,
  StringWriter,
} from "https://deno.land/std@0.177.0/io/mod.ts";
import { copy } from "https://deno.land/std@0.177.0/streams/copy.ts";

const w = new StringWriter("base");
const r = new StringReader("0123456789");
await copyN(r, w, 4); // copy 4 bytes

// Number of bytes read
console.log(w.toString()); //base0123

await copy(r, w); // copy all
console.log(w.toString()); // base0123456789

Output:

base0123
base0123456789