how to draw shapes in a PDF using NodeJS - node.js

Question

I have some existing PDF files and what I want is to highlight some content by overlaying circles or straight lines. I've looked at some NodeJS PDF libraries but couldn't find a solution (some libraries allow creating a PDF from scratch and draw into it; other libraries can modify existing PDFs, but do not support drawing).
A (Linux / OSX) command line solution (e.g. using ImageMagick or some other library) would be perfectly fine, too.
Edit I've since found out that with Image/GraphicsMagick I can in fact do sth. like gm convert -draw "rectangle 20,20 150,100" xxx.pdf[7] xxx2.pdf, but this (1) either draws on all pages or else only on a single one, but then the resulting PDF will only contain that page; (2) the output PDF will contain a bitmap image where I would prefer a PDF with text content.
Edit I've just found HummusJS which is a NodeJS library to manipulate PDF files via declarative JSON objects. Unfortunately, apart from the scrace documentation, the unwieldy API (see below), the tests fail consistently and across the board with Unable to create PDF file, make sure that output file target is available.
completely OT not sure what it is that makes people think such utterly obfuscated APIs are better than simple ones:
var settings = {modifiedFilePath:'./output/BasicJPGImagesTestPageModified.pdf'}
var pdfWriter = hummus.createWriterToModify('./TestMaterials/BasicJPGImagesTest.PDF',settings);
var pageModifier = new hummus.PDFPageModifier(pdfWriter,0);
pageModifier.startContext().getContext().writeText('Test Text', ...
...
var copyingContext = inPDFWriter.createPDFCopyingContextForModifiedFile();
var thirdPageID = copyingContext.getSourceDocumentParser().getPageObjectID(2);
var thirdPageObject = copyingContext.getSourceDocumentParser().parsePage(2).getDictionary().toJSObject();
var objectsContext = inPDFWriter.getObjectsContext();
objectsContext.startModifiedIndirectObject(thirdPageID);
var modifiedPageObject = inPDFWriter.getObjectsContext().startDictionary();

Answer 1

A couple of helpers with HummusJS, to assist with what you are trying to do:
Adding content to existing pages - https://github.com/galkahana/HummusJS/wiki/Modification#adding-content-to-existing-pages
Draw shapes - https://github.com/galkahana/HummusJS/wiki/Show-primitives
using both, this is how to add a circle at 'centerx,centery' with 'radius' and border width of 1, to the first page of 'myFile.pdf'. The end result, in this case will be placed in 'modifiedCopy.pdf':
var pdfWriter = hummus.createWriterToModify(
'myfile.pdf',
{modifiedFilePath:'modifiedCopy.pdf'});
var pageModifier = new hummus.PDFPageModifier(pdfWriter,0);
var cxt = pageModifier.startContext().getContext();
cxt.drawCircle(
centerX,
centerY,
radius,
{
type:stroke,
width:1,
color:'black'
});
pageModifier.endContext().writePage();
pdfWriter.end();
General documentation - https://github.com/galkahana/HummusJS/wiki
If the tests fail, check that an "output" folder exists next to the script being executed, and that there are permissions to write there.