streamResponseLines is a generator function that streams over a response body (returned by fetch()) line by line. Use it like this: `for await (const line of streamResponseLines(response)) { ... }`.

streamResponseLines.js

/**
 * streamResponseLines is a generator function that streams over a response
 * body (returned by fetch()) line by line. Use it like this:
 * `for await (const line of streamResponseLines(response)) { ... }`.
 *
 * @param {Response} response
 */
async function* streamResponseLines(response) {
  const reader = response.body.getReader();
  // Assume the response body is in UTF-8 encoding.
  const textDecoder = new TextDecoder("utf-8");
  let line = "";
  let chunk = new Uint8Array();
  // Read from the response body in chunks.
  for (let readResult = await reader.read(); !readResult.done; readResult = await reader.read()) {
    if (chunk.length > 0) {
      // We have a carryover chunk from a previous iteration. There are
      // guaranteed to be no newlines inside since the previous iteration's
      // chunk.indexOf(10) would have caught it.
      //
      // Stream option has to be true because we haven't encountered a
      // newline yet, more data may be decoded for the current line.
      line += textDecoder.decode(chunk, { stream: true });
    }
    // Get the reader's current chunk.
    chunk = readResult.value;
    // Jump to each newline '\n' byte in the chunk. 10 is the ASCII/UTF-8
    // decimal value of the '\n' byte.
    for (let index = chunk.indexOf(10); index >= 0; index = chunk.indexOf(10)) {
      // We found a newline, decode everything up to this index and consider
      // that as a complete line and yield it.
      line += textDecoder.decode(chunk.subarray(0, index));
      yield line;
      // Reset the line.
      line = "";
      // Shorten the chunk to exclude what we have already decoded.
      chunk = chunk.subarray(index + 1);
    }
  }
  // Flush any remainder bytes in the chunk.
  if (chunk.length > 0) {
    line += textDecoder.decode(chunk);
    yield line;
  }
}

const response = await fetch("https://example.org/some/api/call");
for await (const line of streamResponseLines(response)) {
  console.log(line);
}