If we were writing like foo equals true, we would have wrote down the Boolean literal instead of the numerical one, or strings or whatever, just for you to understand. While it's quite easy to understand this tree, I guess this one is a bit more challenging to deal with. When reading an abstract syntax tree, we call this process traversing the tree, and it's quite complex on, of course, complex programs.
But luckily, many tools such as Babel, ESLint, Prettier, provide us a nice interface for traversing the tree, which is called the visitor pattern. In that case, with Babel, we are saying, okay, dear Babel, please give me all the variable declarations, or the identifiers, or the numerical literals, and we just print them to the console.
Once we get, for example, all the variable declarations, we can start transforming them. So that's the case. We say, okay, please give me all the variable declarations inside of our code. And if the kind of the variable is a var, just transform it into a let, which is not wise. I'm not suggesting you doing that, but it's a very straightforward example. So that makes things easier for us. So that said, on the left, we have our input, var foo equals 10, and on the right, we have our output, let foo equals 10. As you can see, on line two, we have a constant, and we leave it untouched because it's not a variable. It's a const.
So as you can see on the code, we are not saying if it's constant, please change it to let. So we are not doing that. As I said, the same concept applies to other tools, such as Prettier, ESLint, Babel, JS, Codeshift, and we're going to see them in details right now. With ESLint, we have a similar approach. We say, okay, please, ESLint, give me all the template literals you can find in my code. And if I find one, I can report to ESLint an error, such as, please do not use template literals.
And I can also provide a fix for it, but in that case, if I have an expression inside my template literals, I can say, okay, I'm too lazy for that, so I'm just returning, I'm not fixing this. But if I don't have an expression, I can say, okay, so I don't have expression, I don't have an expression, so no dynamic data inside my template literals, so just change the literals into double quotes. So that's what we are doing now. And on the left, the input, on the right, the output. JS Codeshift, it's another awesome example.
It's a library built by Facebook and maintained by Facebook itself, and it's meant for big refactors, large-scale refactors. And the concept is quite similar. Here we are saying, okay, please, JS Codeshift, find all the identifiers. And if the identifier names contains the old name string, replace it with new name string. This is the case, again, for large-scale refactors. So we have like old name factory, which get refactored into new name factory.
So I really want to thank you all for being here. It has been a real pleasure to talk at Node Congress. And these are my social media handlers. So if you want to reach out, I'd be glad to talk with you again about how to refactor code bases and making code transformations. That's what I love to do and to talk about. So thank you again. I hope to see you all soon.