transform-stream.ts

const fileStream = request.body; // Stream of the file content
const reader = fileStream.getReader();
const decoder = new TextDecoder();

let firstColumnValues = [];
let leftover = ''; // Store any partial lines

const processChunk = async ({ done, value }) => {
  if (done) {
    // Process any remaining line in leftover
    if (leftover) {
      const columns = leftover.split(',');
      firstColumnValues.push(columns[0]);
    }
    return;
  }

  const chunk = decoder.decode(value, { stream: true });
  const lines = (leftover + chunk).split('\n');
  leftover = lines.pop(); // Save the last partial line

  for (const line of lines) {
    const columns = line.split(',');
    if (columns.length > 0) {
      firstColumnValues.push(columns[0]); // Extract the first column
    }
  }

  return reader.read().then(processChunk);
};

await reader.read().then(processChunk);