Fix Error.prototype.toString to be ES5 conform.

test/mjsunit/cyclic-error-to-string.js

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 // Test printing of cyclic errors which return the empty string for
 // compatibility with Safari and Firefox.
 

[Lasse Reichstein, 2011/10/26 08:50:44]

So we have a test of Error.toString that tests that it does what it should in
the non-cyclic case?
I.e., that it does the correct conversion using ToString if not undefined for
name and message (and that, e.g., null isn't treated the same as undefined).

[Michael Starzinger, 2011/10/26 09:30:20]

Done. I have renamed this test case to error-tostring.js, because it now does
way more than just testing the cyclic case.

 var e = new Error();
 assertEquals('Error', e + '');

[Lasse Reichstein, 2011/10/26 07:40:24]

I'd use 
 String(e)
to convert an object to a string. More readable.
In this case, we are testing the toString function, so e.toString() is probably
even better here.

[Michael Starzinger, 2011/10/26 09:30:20]

Done.

 
 e = new Error();
 e.name = e;
 e.message = e;
 e.stack = e;
 e.arguments = e;

[Lasse Reichstein, 2011/10/26 07:40:24]

Why .stack and .arguments? They don't seem to affect (and should not be expected
to affect) the toString function.
It would be better if they actually contained string values that should not
occur in the result. I.e. change them to:
  e.stack = "Does not occur in output";
  e.arguments = "Does not occur in output";
  e.type = "Does not occur in output";
(adding .type just to be sure).

[Michael Starzinger, 2011/10/26 09:30:20]

Done.

-assertEquals(': ', e + '');
+assertEquals('', e + '');

[Lasse Reichstein, 2011/10/26 07:40:24]

Seems a little odd that we can get ": " when neither name nor message is the
empty string. I can see that we need to do something, but I'd prefer putting
something else than the empty string in place of recurring objects in the
toString call graph. E.g. "#<error>". But we match Safari, so let's keep it for
now.

[Michael Starzinger, 2011/10/26 09:30:20]

I agree, having a more sensible string representation for the cyclic case would
be good. But since it is not specified how to treat cycles, I think staying in
sync with Firefox (and probably Safari once they fix the ES5 issue here) is a
good thing.

 
 e = new Error();
 e.name = [ e ];
 e.message = [ e ];
 e.stack = [ e ];
 e.arguments = [ e ];
-assertEquals(': ', e + '');
+assertEquals('', e + '');

[Lasse Reichstein, 2011/10/26 07:40:24]

Add a test to check the sequence of visible operations of toString, e.g.,

var seq;
function testSeq(name, message) {
 var e = {
   get name() { 
     seq.push(1); 
     return (name === undefined) ? name : {
         toString: function() { seq.push(2); return name; }; 
     };
   }, 
   get message() {
     seq.push(3);
     return message === undefined ? message : {
       toString: function() { seq.push(4); return message; }
     };
   }
 };
 return Error.prototype.toString.call(e);
}

seq = [];
assertEquals("Error", testSeq(undefined, undefined));
assertEquals([1,3], seq);
seq = [];
assertEquals("e1", testSeq("e1", undefined));
assertEquals([1,2,3], seq);

seq = [];
assertEquals("e2", testSeq(undefined, "e2"));
assertEquals([1,3,4], seq);

seq = [];
assertEquals("e1: e2", testSeq("e1", "e2"));
assertEquals([1,2,3,4], seq);

(not tested).

[Michael Starzinger, 2011/10/26 09:30:20]

Done (added with slight modifications).