Convert an in-ordered and pre-ordered array to a BST
A few key points:


Make sure to put in semicolons!
You don't need to export properties (i.e do this.myVar) if you're not going to access them - this can lead to potentially slowing down recurrence, or in some cases, security vulnerabilities.
If you're going to use arrow functions, keep it consistent - do const Tree = () =&gt; { on the first line


Some more specific things about the actual functionality/logic:


By tracking this.level, you could simplify all required actions into an array by simply pushing their position in the tree. If this.level === 0, you know you are at the top of the tree, and you know to end recurrence.
Using a position-based tracking lets you make it asyncronous, and print the result at the end. Remember that in JS, console.logs take a lot of time, so you want to avoid it as much as possible.
This would let you essentially JSONify the tree into a bunch of function references, which, when called, would populate themselves. Giving the option to return this would make the class more integratable into other things, where a console output is not necessarily the most useful thing. Further, it would let you cut out the repetition of if (left_tree.length&gt;1){, which is not DRY.