I was able to figure it out myself. The issue was that I didn't set up the YouTube iframe API properly. Once I got that in place the rest of the code worked as I expected it to. For reference here is the documentation on the YouTube iframe API.
My working code is as follows:
library(shiny)
library(shinyjs)
ui = shinyUI(fluidPage(
useShinyjs(),
headerPanel("New Application"),
sidebarPanel(
actionButton("getTime","Get Video Time")
)
,mainPanel(
uiOutput("video")
)
))
server = function(input, output) {
output$video <- renderUI({
HTML(
'<html>
<body>
<iframe id="existing-iframe"
width="640" height="360"
src="https://www.youtube.com/embed/fmuUQCB3pAE?enablejsapi=1"
frameborder="0"
></iframe>
<script type="text/javascript">
var tag = document.createElement(\'script\');
tag.src = \'https://www.youtube.com/iframe_api\';
var firstScriptTag = document.getElementsByTagName(\'script\')[0];
firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);
var player;
function onYouTubeIframeAPIReady() {
player = new YT.Player(\'existing-iframe\');
}
</script>
</body>
</html>'
)
})
observeEvent(input$getTime,{
runjs("alert(player.getCurrentTime())")
})
}
shinyApp(ui = ui, server = server)
Note the major difference is inside the HTML() wrapper beginning on line 18. I set up the iframe the same way but with id="existing-iframe" along with enablejsapi=1 inside the URL. Then I needed the <script> tag below which had all of the JavaScript code I needed to get the app to recognize that I was using the YouTube iframe api. I also cleaned up the code inside the runjs() wrapper on line 44.