add state borders to the output of leaflet where the input is counties shapefile

Question

I downloaded the county shapefile off of data.gov. I can use them in leaflet as follows to plot counties in WA, ID and OR. I would like to make the state borders thicker. How can I do that?

counties <- readOGR(paste0(dir, "/tl_2017_us_county.shp"),
                    layer = "tl_2017_us_county", GDAL1_integer64_policy = TRUE)

counties <- counties[counties@data$STATEFP %in% c("16", "41", "53"), ]
counties <- subset(counties, STATEFP %in% c("16", "41", "53") )
counties <- rmapshaper::ms_simplify(counties)

counties %>%
leaflet() %>%
setView(lng = -118.4942, lat = 47.2149, zoom = 5) %>%
addPolygons( fillColor = "green", fillOpacity = 0.5,
             color = "black", opacity = 1.0, weight = .6, smoothFactor = 0.5,
             highlightOptions = highlightOptions(color="white", weight=2, bringToFront = TRUE),
             label= ~ NAME) 

P.S. I would like to do this in R studio, I do not know any of the advanced codings in here: https://leafletjs.com/

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Edit The goal was to make the states to be distinguishable, which I did with the following:

factpal <- colorFactor(topo.colors(5), counties$category)

counties %>%
leaflet() %>%
# addTiles() %>% 
setView(lng = -118.4942, lat = 47.2149, zoom = 5) %>%
addPolygons( fillColor = ~factpal(STATEFP), fillOpacity = 0.5,
             # The following line is associated with borders
             color = "black", opacity = 1.0, weight = .6, smoothFactor = 0.5,
             highlightOptions = highlightOptions(color="white", weight=2, bringToFront = TRUE),
             label= ~ NAME)

I still would like to know the answer of making borders thicker.

Answer 1

There is probably no in-built option to achieve this, since you're trying to plot the outline of all county-polygons belonging to a state. It works of course when you highlight the states by coloring all polygons inside it.

To achieve what you want with the sp-package you can do this:

library(sp)
library(leaflet)

states <- aggregate(counties[, "STATEFP"], by = list(ID = counties@data$STATEFP), 
                    FUN = unique, dissolve = T)

counties %>%
  leaflet() %>%
  setView(lng = -118.4942, lat = 47.2149, zoom = 5) %>%
  addPolygons( fillColor = "green", fillOpacity = 0.5,
               color = "black", opacity = 1.0, weight = .6, smoothFactor = 0.5,
               highlightOptions = highlightOptions(color="white", weight=2, bringToFront = TRUE),
               label= ~ NAME) %>%
  addPolylines(data = states, color = "black", opacity = 1, weight = 3)