Arrow function and .this context

console.log('Global this', this)

personA = {
  'name': 'Stella',
  'yob': 1986,
  'age1': function() {
    console.log('Age1 is:', 2020 - this.yob)
  },
  'age2': () => {console.log('Age2 is:', this.yob)}
}

personA.age1() // output: 34s
personA.age2() // output: undefined

function personB() {
  this.name = 'Thomas'
  this.yob = 1991

  this.age3 = function() {console.log('Age3 is:', 2020 - this.yob)}

  this.age4 = () => {console.log('Age4 is:', 2020 - this.yob)}
}

personX = new personB()
personX.age3() // output: 29
personX.age4() // output: 29