NodeJS: Merge two PDF files into one using the buffer obtained by reading them

Question

I am using fill-pdf npm module for filling template pdf\'s and it creates new file which is read from the disk and returned as buffer to callback. I have two files for which

Answer 1

Here's what we use in our Express server to merge a list of PDF blobs.

const { PDFRStreamForBuffer, createWriterToModify, PDFStreamForResponse } = require('hummus');
const { WritableStream } = require('memory-streams');

// Merge the pages of the pdfBlobs (Javascript buffers) into a single PDF blob                                                                                                                                                                  
const mergePdfs = pdfBlobs => {
  if (pdfBlobs.length === 0) throw new Error('mergePdfs called with empty list of PDF blobs');
  // This optimization is not necessary, but it avoids the churn down below                                                                                                                                                
  if (pdfBlobs.length === 1) return pdfBlobs[0];

  // Adapted from: https://stackoverflow.com/questions/36766234/nodejs-merge-two-pdf-files-into-one-using-the-buffer-obtained-by-reading-them?answertab=active#tab-top                                                     
  // Hummus is useful, but with poor interfaces -- E.g. createWriterToModify shouldn't require any PDF stream                                                                                                              
  // And Hummus has many Issues: https://github.com/galkahana/HummusJS/issues                                                                                                                                              
  const [firstPdfRStream, ...restPdfRStreams] = pdfBlobs.map(pdfBlob => new PDFRStreamForBuffer(pdfBlob));
  const outStream = new WritableStream();
  const pdfWriter = createWriterToModify(firstPdfRStream, new PDFStreamForResponse(outStream));
  restPdfRStreams.forEach(pdfRStream => pdfWriter.appendPDFPagesFromPDF(pdfRStream));
  pdfWriter.end();
  outStream.end();
  return outStream.toBuffer();
};

module.exports = exports = {
  mergePdfs,
};