Encountering a TypeError while attempting to load a PDF document using PDF-LIB on Node

Question

Encountering a TypeError while attempting to load a PDF document using PDF-LIB on Node

As part of a project, I am required to make edits to PDF files. To achieve this, I have been using PDF Lib and have had success with it so far. However, I've encountered some difficulties when trying to load an existing PDF file which is evident in the following error message:

TypeError: pdf must be of type string or Uint8Array or ArrayBuffer, but was actually of type undefined

Below is the code snippet that triggers this error:

const pdfData = await fs.readFile('./test.pdf', 'utf8', (err, data) => {
    if (err) {
        console.error(err);
        return;
    }
});
const pdfDoc = await PDFDocument.load(pdfData);

Has anyone else come across this same issue?

javascript typescript pdf

Answer 1

Answer №1

The issue you are facing is that fs.readFile is an asynchronous API that does not return a value. To resolve this, rather than importing / requiring fs, import fs/promises:

import promiseBasedFs from 'node:fs/promises';
// Instead of
// import fs as oldStyleFs from 'node:fs';

In the new /promises version of the API, fs.readFile returns a promise (and no longer requires a callback):

const pdfData = await promiseBasedFs.readFile(yourFilePath, 'utf-8');
const pdfDoc = await PDFDocument.load(pdfData);

If for some reason you cannot use the /promises version, you can "lift" the call into a Promise using either util.promisify:

const promisifiedReadFile = util.promisify(fs.readFile);
const pdfData = await promisifiedReadFile(yourFilePath, 'utf-8');

or by explicitly wrapping the call to readFile in a Promise constructor:

const pdfData = await new Promise((resolve, reject) => {
  oldStyleFs.readFile(yourFilePath, 'utf-8', (err, data) => {
    if (err) reject(err);
    else resolve(data);
  });
});

Alternatively, you could switch to callbacks entirely and move your PDFDocument.load call inside the callback (not recommended):

fs.readFile(yourFilePath, 'utf-8', async (err, pdfData) => {
  if (err) { /* handle error */ }
  const pdfDoc = await PDFDocument.load(pdfData);
});

Answer 2

The issue you are facing is that fs.readFile is an asynchronous API that does not return a value. To resolve this, rather than importing / requiring fs, import fs/promises:

import promiseBasedFs from 'node:fs/promises';
// Instead of
// import fs as oldStyleFs from 'node:fs';

In the new /promises version of the API, fs.readFile returns a promise (and no longer requires a callback):

const pdfData = await promiseBasedFs.readFile(yourFilePath, 'utf-8');
const pdfDoc = await PDFDocument.load(pdfData);

If for some reason you cannot use the /promises version, you can "lift" the call into a Promise using either util.promisify:

const promisifiedReadFile = util.promisify(fs.readFile);
const pdfData = await promisifiedReadFile(yourFilePath, 'utf-8');

or by explicitly wrapping the call to readFile in a Promise constructor:

const pdfData = await new Promise((resolve, reject) => {
  oldStyleFs.readFile(yourFilePath, 'utf-8', (err, data) => {
    if (err) reject(err);
    else resolve(data);
  });
});

Alternatively, you could switch to callbacks entirely and move your PDFDocument.load call inside the callback (not recommended):

fs.readFile(yourFilePath, 'utf-8', async (err, pdfData) => {
  if (err) { /* handle error */ }
  const pdfDoc = await PDFDocument.load(pdfData);
});

Encountering a TypeError while attempting to load a PDF document using PDF-LIB on Node

Answer №1

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