An example to show how to highlight polygons on hover using onRender

library(sp)

## Warning: package 'sp' was built under R version 3.3.2

## Loading required package: methods

library(albersusa)
library(maptools)

## Checking rgeos availability: TRUE

library(leaflet)

spdf <- rmapshaper::ms_simplify(usa_composite())

pal <- colorNumeric(palette = "Blues", domain = spdf@data$pop_2014)
pal2 <- colorNumeric(palette = "Reds", domain = spdf@data$pop_2013)

bounds <- c(-125, 24 ,-75, 45)

leaflet(
  options=
    leafletOptions(
      worldCopyJump = FALSE,
      crs=leafletCRS(
        crsClass="L.Proj.CRS",
        code='EPSG:2163',
        proj4def='+proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs',
        resolutions = c(65536, 32768, 16384, 8192, 4096, 2048,1024, 512, 256, 128)
      ))) %>%
  fitBounds(bounds[1], bounds[2], bounds[3], bounds[4]) %>%
  setMaxBounds(bounds[1], bounds[2], bounds[3], bounds[4]) %>%
  addPolygons(data=spdf, weight = 1, color = "#000000",
              fillColor=~pal(pop_2014),
              fillOpacity=0.7,
              label=~stringr::str_c(
                name,' ',
                formatC(pop_2014, big.mark = ',', format='d')),
              labelOptions= labelOptions(direction = 'auto'),
              highlightOptions = highlightOptions(
                color='#ff0000', opacity = 1, weight = 2, fillOpacity = 1,
                bringToFront = TRUE, sendToBack = TRUE),
              group="2014") %>%
  addPolygons(data=spdf, weight = 1, color = "#000000",
              fillColor=~pal2(pop_2013),
              fillOpacity=0.7,
              label=~stringr::str_c(
                name,' ',
                formatC(pop_2014, big.mark = ',', format='d')),
              labelOptions= labelOptions(direction = 'auto'),
              highlightOptions = highlightOptions(
                color='#00ff00', opacity = 1, weight = 2, fillOpacity = 1,
                bringToFront = TRUE, sendToBack = TRUE),
              group="2013") %>%
  addLayersControl(
    baseGroups=c("2014", "2013"),
    position = "topleft",
    options = layersControlOptions(collapsed = FALSE)
  )