use

turndownService.use(plugin)
turndownService.use([plugin1, plugin2, ...])

var TurndownService = require('turndown')
var turndownPluginGfm = require('turndown-plugin-gfm')

var turndownService = new TurndownService()
var gfm = turndownPluginGfm.gfm

turndownService.use(gfm)

var turndownPluginGfm = require('turndown-plugin-gfm')
var tables = turndownPluginGfm.tables
var strikethrough = turndownPluginGfm.strikethrough

turndownService.use([tables, strikethrough])

function myPlugin(turndownService) {
  turndownService.addRule('underline', {
    filter: 'u',
    replacement: function (content) {
      return '__' + content + '__'
    }
  })
  
  turndownService.addRule('highlight', {
    filter: 'mark',
    replacement: function (content) {
      return '==' + content + '=='
    }
  })
}

turndownService.use(myPlugin)

turndownService
  .use(gfm)
  .keep(['sub', 'sup'])
  .remove('style')

function configurablePlugin(options) {
  return function (turndownService) {
    turndownService.addRule('custom', {
      filter: options.filter,
      replacement: function (content) {
        return options.prefix + content + options.suffix
      }
    })
  }
}

turndownService.use(configurablePlugin({
  filter: 'kbd',
  prefix: '`',
  suffix: '`'
}))

var plugins = [
  turndownPluginGfm.tables,
  turndownPluginGfm.strikethrough,
  turndownPluginGfm.taskListItems
]

turndownService.use(plugins)

function myPlugin(turndownService) {
  // Add rules
  turndownService.addRule('ruleName', { /* rule definition */ })
  
  // Keep elements
  turndownService.keep(['element1', 'element2'])
  
  // Remove elements
  turndownService.remove('element3')
  
  // Modify options
  turndownService.options.customOption = 'value'
}