How to enable SNAP function

How to enable snapping function?. A feature where I can use area, perimeter tool to draw accurately.
I have referred the documentation -

Here is my small code snippet which I have tried to do.

tool = this.viewerInstance.docViewer.getTool('AnnotationCreateAreaMeasurement');

Another confusion is the setSnapMode(mode). Here mode is a number so what is the exact parameter to pass in setSnapMode. Also will it work with or without fullAPI? if no then how to enable fullAPI ?
I have tried this in setting this true in WebViewer({ })
fullAPI: true.
But getting this error - Error: "fullAPI" is not a valid constuctor option for PDF.js Express

Hi there!

Snapping is not supported in PDF.js Express. A lot of our documentation is shared with our sister product, PDFTron WebViewer, and we must have forgot to exclude that API from Express.