The remove function of the AVL tree appears to delete the entire tree by setting the root property of the tree to undefined if the root value is removed.

This is because the function's call this.root = balanceUpstream( node.parent ); has parameter node.parent = null if node is the root (see commented out function call below). Function balanceUpstream will then immediately return undefined which remove assigns to this.root deleting the tree.

I have fixed this in my implementation of avl-tree.js using a ternary operator:

remove( value ) {
        const node = super.find( value );
        if ( node ) {
            const found = super.remove( value );
            const parent = node.parent ? node.parent : this.root;
            this.root = balanceUpstream( parent );
            //this.root = balanceUpstream( node.parent );
            return found;
        }
        return false;
    }

Posted as a (potential) issue as I am not sure whether this is an issue only in my implementation which differs somewhat from the one in this repository.

Cheers!