MobX autorun and reaction inside constructor

Do autorun and reaction have to be inside the constructor in order to work? Can I write this simple example without constructor?

Also the code that I have inside the autorun runs normally but if I change it to console.log(this.expenses) it doesn't work. Why is that?

import { observable, action, computed, useStrict, autorun, reaction } from 'mobx'
useStrict(true)

class ExpensesStore {

  @observable.shallow expenses = []

  @action addExpense = (expense) => {
   this.expenses.push(expense)
  }

  @computed get getExpense() {
    if(this.expenses.length > 0) {
      return `This is computed from ${this.expenses[0] + this.expenses[1]}`
    }

  }


  constructor() {
    autorun(() => {
      console.log(`${this.expenses}`)
    })

    reaction(
      ()=>this.expenses.map(expense => expense), expense => console.log(expense)
    )
  }


}

const store = window.store= new ExpensesStore()

export default store

autorun and reaction does not have to be in the constructor. You could for example do this if you prefer:

Example

class ExpensesStore {
  @observable.shallow expenses = []

  @action addExpense = (expense) => {
   this.expenses.push(expense)
  }

  @computed get getExpense() {
    if(this.expenses.length > 0) {
      return `This is computed from ${this.expenses[0] + this.expenses[1]}`
    }
  }
}

const store = new ExpensesStore()

autorun(() => {
  console.log(`${store.expenses}`)
})

reaction(
  () => store.expenses.map(expense => expense), expense => console.log(expense)
)

The reason why console.log(`${this.expenses}`) works and console.log(this.expenses) doesn't is because you are not dereferencing anything in your shallow array when you write this.expenses.

When you write...

`${this.expenses}`

... you are implicitly calling toString() on this.expenses. You can use toJS or slice to get the same effect when not putting it in a string:

Example (JSBin)

class ExpensesStore {
  @observable.shallow expenses = []

  @action addExpense = (expense) => {
   this.expenses.push(expense)
  }

  @computed get getExpense() {
    if(this.expenses.length > 0) {
      return `This is computed from ${this.expenses[0] + this.expenses[1]}`
    }
  }
}

const store = new ExpensesStore()

autorun(() => {
  // store.expenses.slice() works just as well
  console.log(toJS(store.expenses))
})

reaction(
  () => store.expenses.map(expense => expense), expense => console.log(expense)
)

setTimeout(() => {
  store.addExpense(1000)
}, 1000)