app.js

const person = {
  firstName: 'Martin',
  lastName: 'Laws',
  heightInCm: 188,
  heightInInches: function() {
    return this.heightInCm / 2.54
  },
  speak() {
    console.log(`Hi, my name is ${this.firstName}!`)
    return
  },
  friends: []
}

person.heightInInches()

person.js

function Person(firstName, lastName, heightInCm) {
  this.firstName = firstName
  this.lastName = lastName
  this.heightInCm = heightInCm
  this.friends = []

  this.addFriend = function(friend) {    
    if (typeof friend === 'object') {
      this.friends.push(friend)
    } else {
      console.log('GO GET A REAL FRIEND!')
    }
  }

  this.fullName = function() {
    console.log(`${this.firstName} ${this.lastName}`)
  }

  this.heightInInches = function() {
    return this.heightInCm / 2.54
  }
  
  this.speak = function() {
    console.log(`Hi, my name is ${this.firstName}!`)
    return
  }
}

const martin = new Person('Martin', 'Laws', 188)
const scott = new Person('Scott', 'Morrison', 189)

martin.addFriend(scott)
martin.addFriend(123)

// scott.friends.push(martin)

console.log('Martin:', martin)
console.log('Scott:', scott)