Accepted answer

Try to bind the function inside the constructor like this:

this.isLetterInWord = this.isLetterInWord.bind(this);
this.fillWord = this.fillWord.bind(this)
this.handleGuess = this.handleGuess.bind(this);


Here you are defining class properties, which is currently (I think) is stage-3 proposal. In other words it is not supported by default. May be because of this you are getting errors. You can use class-properties-transform plugin for Babel to add support of this syntax though if you want to .

isLetterInWord = (letter) => {

  fillWord = () => {


Try defining methods as and see if it works

isLetterInWord(letter) {


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