Typescript is a Java-like Object oriented language which also accepts java-script.
What that means is that When Typescript is compiled, it is first compiled into Javascript, which then is interpreted, JavaScript is a subset of Typescript, you can write Javascript code inside a typescript file and it is perfectly OK.
I think learning JavaScript is a big help for Typescript learners, because since you're learning Typescript, I assume that you want ot, or will end up working on Angular2+ in which case you will most likely interact with some front-end Javascript code.
Keep in mind that Typescript is a little more challenging than JavaScript, especially if you have not worked on common languages like
Java and C++ before, but I think it is not challeging in the sense of the language itself, but the framework it uses (Angular2+) have a certain approach/ structure which may take some time to learn, not Typescript itself.
So, Yes learning JavaScript is a big help, it allows you to use Javascript in combination with Typescript (not a very popular practice among Angular2+ programmers, but it is used often). Java is also helpful in terms of knowing the object oriented part of Typescript.