//captcha 
let captcha = require('canvas-captcha')
let captchaOptions = {
  charPool: ('abcdefghijklmnopqrstuvwxyz' + 'abcdefghijklmnopqrstuvwxyz'.toUpperCase() + '1234567890').split('') //char pool Array
  ,size: {
    width: 100
    ,height: 32
  } //image size
  ,textPos: {
    left: 15
    ,top: 26
  } //text drawing start position
  ,rotate: .01 //text ratate
  ,charLength: 4 //how many chars
  ,font: '26px Unifont' //font size
  ,strokeStyle: '#0088cc' //style
  ,bgColor: '#eeeeee' //bg color
  ,confusion: true //draw another group background text to mangle the text
  ,cFont: '30px Arial' //bg text style
  ,cStrokeStyle: '#adc' //bg text color
  ,cRotate: -.05 //bg text rotate
}

//callback style
app.get('/captcha', function(req, res) {
  captcha(captchaOptions, function(err, data) {
    if(err) {
      res.send(err)
    }
    else {
      req.session.captcha = data.captchaStr
      res.end(data.captchaImg)
    }
  })
})

//use promise
let captchaPromise = function(options) {
  return new Promise(function(resolve, reject) {
    captcha(options, function(err, data) {
      if(err) reject(err)
      else resolve(data)
    })
  })
}

//in express
app.get('/captcha', function(req, res) {

  captchaPromise(captchaOptions)
  .then(function(data) {
    req.session.captcha = data.captchaStr
    res.end(data.captchaImg)
  }, function(err) {
    res.send(err)
  })

})

//in koa
app.get('/captcha', function* (next) {

  var data = yield captchaPromise(captchaOptions)
  this.session.captcha = data.captchaStr
  this.body = data.captchaImg

})

//or
app.get('/captcha', async ctx => {

  let data = await captchaPromise(captchaOptions)
  ctx.session.captcha = data.captchaStr
  ctx.body = data.captchaImg

})