const context = canvasElgetContext2d; const dpr = windowdevicePixelRatio 1 const bsr = contextwebkitBackingStorePixelRatio contextmozBackingStorePixelRatio

This code snippet is setting up the canvas element to render a PDF page.

Here's a breakdown of what each line does:

const context = canvasEl.getContext("2d");: Get the 2D rendering context of the canvas element.
const dpr = window.devicePixelRatio || 1: Get the device pixel ratio of the window or default to 1.
const bsr = context.webkitBackingStorePixelRatio || context.mozBackingStorePixelRatio || context.msBackingStorePixelRatio || context.oBackingStorePixelRatio || context.backingStorePixelRatio || 1;: Get the backing store pixel ratio from the rendering context or default to 1.
const outputScale = dpr / bsr;: Calculate the output scale by dividing the device pixel ratio by the backing store pixel ratio.
const deviceWidth = window.screen.width;: Get the width of the device screen.
const deviceHeight = window.screen.height;: Get the height of the device screen.
canvasEl.width = 595;: Set the width of the canvas element to 595 pixels.
canvasEl.height = 842;: Set the height of the canvas element to 842 pixels.
var scale = canvasEl.width / page.getViewport({scale: 1}).width;: Calculate the scale factor based on the width of the canvas element and the width of the PDF page viewport at scale 1.
var viewport = page.getViewport({scale}): Get the viewport of the PDF page using the calculated scale factor.
const renderContext = { canvasContext: context, viewport };: Create a render context object with the canvas context and the viewport.
page.render(renderContext);: Render the PDF page onto the canvas using the render context.