How to use an I2C chip? (Adafruit MMA8451)

The write...read pattern seems right. If you check out the libraries for some of the existing modules you'll see it's done in that way (http://www.espruino.com/I2C#using-i2c then you can usually go to a page a find a link to the module there). For instance in http://www.espruino.com/modules/VL53L0X.js there are two functions r and w:

VL53L0X.prototype.r = function(addr,n) {
  this.i2c.writeTo(this.ad, addr);
  return this.i2c.readFrom(this.ad, n);
};
VL53L0X.prototype.w = function(addr,d) {
  this.i2c.writeTo(this.ad, addr, d);
};

What you might be missing is some chips only read properly if you do an I2C write and read without something called a "stop" between. You can force that my changing the address var to an object as in: http://www.espruino.com/Reference#l_I2C_writeTo

In your case maybe:

    this.i2c.writeTo({address:this.MMA8451_ADDR, stop:false}, this.MMA8451_REG_OUT_X_MSB);
    let buffer = this.i2c.readFrom(this.MMA8451_ADDR, 7);