src/third_party/mozjs-45/js/src/jit-test/tests/debug/Source-url.js

// Source.prototype.url can be a string or null.

var g = newGlobal();
var dbg = new Debugger;
var gw = dbg.addDebuggee(g);
for (var fileName of ['file:///var/foo.js', null]) {
    g.evaluate("function f(x) { return 2*x; }", {fileName: fileName});
    var fw = gw.getOwnPropertyDescriptor('f').value;
    assertEq(fw.script.source.url, fileName);
}