This WebSharper code sample draws a blue rectangle on a canvas.

FillRect.fs

module FillRect =
 
    open IntelliFactory.WebSharper
    open IntelliFactory.WebSharper.Html
    open IntelliFactory.WebSharper.Html5
 
    [<JavaScript>]
    let main() =
        let element = HTML5.Tags.Canvas [Text "Fallback content goes here."]
        let canvas  = As<CanvasElement> element.Dom
        canvas.Height <- 200
        canvas.Width  <- 200
        let context = canvas.GetContext "2d"
        context.FillStyle <- "blue"
        context.FillRect(50., 50., 120., 120.)
        element
                    
    type Control() =
 
        inherit Web.Control()
 
        [<JavaScript>]
        override __.Body = main() :> _