Can't access parent variables from prototype function [duplicate]

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• Arrow function vs function declaration / expressions: Are they equivalent / exchangeable? 2 answers

I have a class in JavaScript which goes roughly as follows:

function StateManager () {
  this.storeDir = { read: (a, b) => { return false } }
  this.state = this.storeDir.read('./state.json', 'json') || { state: 'new' }
}

StateManager.prototype.update = () => {
  console.log(this.state.state)
}

module.exports = new StateManager()

Assuming this is a file called "StateManager.js", and we have another file in the same folder, I should be able to call it as follows:

let StateManager = require('./StateManager')

console.log(StateManager)
StateManager.update()

And I expect to see the StateManager object followed by the current state, which because we're using a dummy function will always be "new". However, for some unknown reason, this within .update() is an empty object and hence this.state.state errors out. StateManager appears to contain a popular StateManager object, however, so I can't understand why this is so. Is there a way I can access this.state within the StateManager.prototype.update function?

via Popey Gilbert