Problem in brief

HtmlUnit/Rhino throws ScriptException: null when a "bound function" is given as the callback function of Promise.then(). This error is caused by cx.topCallScope being null when passed to BoundFunction.call().

More details

This problem was introduced after the implementation of Promise was changed to Rhino's native implementation in HtmlUnit version 2.56.

Testing

Here are some tests to show the behavior of HtmlUnit 2.66 and Chrome.

<!DOCTYPE html>
<html>
<head>
<script>
let f = function () { console.log(this) }
Promise.resolve().then(f.bind(null)); // Chrome prints Window {...}, HtmlUnit throws ScriptException -- bad
Promise.resolve().then(f.bind("")); // Chrome and HtmlUnit prints String {''} -- good
Promise.resolve().then(() => f.bind(null).call()); // Chrome and HtmlUnit prints Window {...} -- good
Promise.resolve().then(() => f.bind('').call()); // Chrome and HtmlUnit prints String {''} -- good
</script>
</head>
<body>
</body>
</html>