Create a new file in your project folder called light-meter.js. Set up your normal scaffolding: requiring in Johnny-Five and Raspi-IO, setting up our Board object, and creating the board.on('ready') handler:

const Raspi = require('raspi-io')
const five = require('johnny-five')

const board = new five.Board({
  io: new Raspi()
})

board.on('ready', () => {
})

Inside the board.on('ready') handler, construct your Servo and Light objects:

let servo = new five.Servo({
  controller: "PCA9685",
  pin: 0
})

let lightSensor = new five.Light({
  controller: 'TLS2561'
})

Then, we need to build a lightSensor.on('change') handler. We're going to use the Sensor.scaleTo([min, max]) to scale the 0-255 input of the light sensor to the 0-180 output of the servo:

lightSensor.on('change', function(){
  servo.to(this.scaleTo([0, 180]))
})

And that's it! Let's get it on the Pi and see it at work.