Adafruit 16-Channel 12-bit PWM/Servo Shield - I2C interface

Great! In this case you should be able to do what you want in normal JavaScript, so actually delving into Espruino itself isn't needed - maybe that link should mention it?

What you really need is: http://www.espruino.com/Writing+Modules

The modules that use I2c (http://www.espruino.com/I2C#using-i2c) are a good start - and the code for them is usually here: https://github.com/espruino/EspruinoDocs/tree/master/devices

To get you started, just copy/paste this into the Web IDE:

Modules.addCached("pca9685_servo",function() {
var C = {
PCA9685_SUBADR1 : 0x2,
PCA9685_SUBADR2 : 0x3,
PCA9685_SUBADR3 : 0x4,
PCA9685_MODE1 : 0x0,
PCA9685_PRESCALE : 0xFE,
LED0_ON_L : 0x6,
LED0_ON_H : 0x7,
LED0_OFF_L : 0x8,
LED0_OFF_H : 0x9,
ALLLED_ON_L : 0xFA,
ALLLED_ON_H : 0xFB,
ALLLED_OFF_L : 0xFC,
ALLLED_OFF_H : 0xFD
};
function PWMServoDriver(i2c) {
  this.i2c = i2c;
  this.addr = 0x40;  
  this.reset(()=>{
    this.setPWMFreq(1000);
  });
}
PWMServoDriver.prototype.write = function(r,d) {
  this.i2c.writeTo(this.addr,r,d);
};
PWMServoDriver.prototype.read = function(r) {
  this.i2c.writeTo(this.addr,r);
  return this.i2c.readFrom(this.addr,1)[0];
};
PWMServoDriver.prototype.reset = function(callback) {
  this.write(C.PCA9685_MODE1, 0x80);
  if (callback) setTimeout(callback,10);
};
PWMServoDriver.prototype.setPWM = function(num,on,off) {
  this.i2c.writeTo(this.addr,
                   C.LED0_ON_L+4*num,
                   on, on>>8,
                   off,off>>8 );
};
PWMServoDriver.prototype.setPWMFreq = function(freq) {
 // FIXME
};
exports.connect = function(i2c) {
  return new PWMServoDriver(i2c);
};
});

var i2c = new I2C(); // software I2C
i2c.setup({sda:SDA_PIN, scl:SCL_PIN});
var servo = require("pca9685_servo").connect(i2c);

So you need to implement setPWMFreq, but the general outline of what you need should be there for the other stuff. I did setPWM because Espruino can do it a bit more tidily than Arduino can.

Other thing to note is that servos don't run at 1000Hz (the default) so you have to use Adafruit's example: https://github.com/adafruit/Adafruit-PWM-Servo-Driver-Library/blob/master/examples/servo/servo.ino

Part of me thinks it's worth just setting it to 60Hz by default and adding a 'setServo' function as well though. Most people are going to use this board/chip for servos anyway :)