How to Type Objects in JavaScript

So let's say the data comes in, I don't know, it's like CSV, JSON. And so I have a bunch of rows coming in and each of them has some sort of shape. But I don't really know what it is yet. So I imagine myself doing next in JavaScript, if I don't knows what those types are, is one of two things. One is just take a guess and say, well, I kind of assume this field is this. And then basically go to step one where I'm like, okay, now we're going to make an assumption based on that.