Could you copy+paste your component, please? Do `console.log(this)` and `console.log(this.textInput)` show what you expected them to?
I would check out the 4 answers here (https://stackoverflow.com/questions/53627803/how-can-i-solve-this-is-undefined-in-react). If binding is the issue, I have had and seen issues in the past of people using `const` inside of class components when they shouldn't, or using `function textInput...` instead of `textInput = () => ...`.
Another StackOverflow answer pointed out that my article is out of date. This article uses class components and now functional components are more common. I need to update this article for functional components!