Making PDF.js work with digital signatures

Out of the box PDF.js doesn’t support displaying signatures on PDF files. This is strange since it’s such a common scenario and it’s been requested many times over the years.

Thankfully there’s a simple solution to this. You need to modify the pdf.worker.js file with the following modification.

In the var WidgetAnnotation = /*#__PURE__*/ function, look for the following code:

if (data.fieldType === 'Sig') {
  data.fieldValue = null;

And comment out the setFlags line like this:

if (data.fieldType === 'Sig') {
  data.fieldValue = null;
  // _this3.setFlags(_util.AnnotationFlag.HIDDEN);

You don’t need to make any modifications to the pdf.js file itself just the pdf.worker.js file.

To do this you download PDF.js from the project page. Take the pdf.worker.js file from the build folder and make the modification. In your web project, use the local modified version of this file but keep referencing the base file through a CDN (or locally if you wish).

Don’t forget to minify your modified worker file afterwards.

Of course, you’ll have to make this manual modification each time you update PDF.js.

Never let stray errors in the console

When doing web development I fix all the errors that show up in the console. Often they don’t seem to impede the actual use of the software, but nevertheless I think it’s a good practice to fix them all. This following example should illustrate why.

I was doing some maintenance on an Angular project at work. I noticed that there were a couple of errors showing up in the console, but the web site still worked great, even with the errors.

The reason I was working on this project was because of a bug that was reported by QA. There was a display error that was happening and it was related to animations that were used in the Angular transitions, these are used when switching from one component to another. The error manifested itself with Android phones and iPhones. This bug probably slipped by because everything was working fine on desktop browsers.

After investigating, I saw that there was an error in the console about calling a property on undefined. On a view, this can be fixed using the ?. operator. The view used the ?. operator elsewhere but there was one place it was missing. I was sure this couldn’t be what was messing up the display of the animated transition.

I searched and searched but couldn’t find the problem.

So I decided that while I was there, I might as well fix this error that showed up in the console. I changed the . operator for ?. and lo and behold this also fixed the display problem.

By correcting it, it also corrected the other problem, which was probably caused by having an error during the Angular rendering.

This example demonstrates that you should never let console errors live, even if they don’t seem to have any negative effect on the web site.

JavaScript tip: format console.log output

You can format the output of console.log() using CSS. Use %c in your console log string and then add an argument for the CSS.

Here’s a simple script that uses CSS formatting to make a formatted console logger:

const fl = {
  log: function (title, message, color) {
    console.log('%c%s:%c %s', 
      `color: ${color}; font-weight: bold; text-decoration: underline ${color}`,
      'color: black',
      title, message);

  info: function (message) {
    this.log('Info', message, 'green');

  warning: function (message) {
    this.log('Warning', message, 'orange');

  error: function (message) {
    this.log('Error', message, 'red');


Here is the result in the console:

console formatting