Web.Page question

Hello Power BI developers team,

I realized that javascript executes inside Power query through Web.Page like following

let
    Source = Web.Page(
        "<script>
        var square = function (x){return x*x;};
        var c = square(12);
      document.write(c);          
        </script>"),
    Data = Source{0}[Data],
    Children = Data{0}[Children],
    #"Expanded Children" = Table.ExpandTableColumn(Children, "Children", {"Kind", "Name", "Children", "Text"}, {"Kind.1", "Name.1", "Children.1", "Text.1"})
in
    #"Expanded Children"

But there are certain things that don't. For e.g. var square changed to const square in the above exmaple

let
    Source = Web.Page(
        "<script>
        const square = function (x){return x*x;};
        var c = square(12);
      document.write(c);          
        </script>"),
    Data = Source{0}[Data],
    Children = Data{0}[Children],
    #"Expanded Children" = Table.ExpandTableColumn(Children, "Children", {"Kind", "Name", "Children", "Text"}, {"Kind.1", "Name.1", "Children.1", "Text.1"})
in
    #"Expanded Children"

One more example of non-execution

//doesnot execute
let
    Source = Web.Page(
        "<script>
        var obj = {foo:'bar,baz:'42'};
        var values = Object.keys(obj);
      document.write(values);          
        </script>"),
    Data = Source{0}[Data],
    Children = Data{0}[Children],
    Children1 = Children{0}[Children]
in
    Children1

I have tried with few and some syntax excutes while other don't.

E.g. Regex positive lookbehind does not work

/(?<=[a-zA-Z]+[/]+[a-zA-Z]+[ ])\d+/gm

but negative  lookahead works.

/\d+(?![a-zA-Z]+[/]+[a-zA-Z]+[ ])/gm

String

123INVOICE 7320210279

Rather than trying to figure out which works and which doesn't is it kindly possible to advise which javascript excution engine is used inside PQWRY. Is it equivalent to an engine <IE9?

If I can have this infomration I can either tune my script to match the version or not use the syntax at all.

Many thanks in advance.