enable typeof "undefined" for general use

[alexlamsl]

Based on #1443 (comment)

/cc @kzc

[kzc]

Can you please add the test from #1443 (comment) to see if it still performs correctly if undefined is redefined? If not, we must keep this transform under the "unsafe" flag.

function f(undefined) {
    return function() {
        var n = 1;
        return typeof n == "undefined";
    };
}
console.log( f(1)() );

It should be tested with and without mangle.

[alexlamsl]

@kzc added your test case (took liberty to remove console.log()) with only comparisons and without mangle.

In terms of code, AST_Undefined seems to always output as void 0.

[kzc]

Could you please add another test undefined_redefined_mangle with mangle enabled?

[kzc]

Regarding void 0 and undefined, all these tests should use expect_exact instead of expect because the test result is subject to parsing which may not be in the form you think it is in. It's hard to explain.

[alexlamsl]

@kzc I missed the "with or" bit in your previous comment - sorry about that!

As for expect_exact instead of expect - are you worried that the parser might get in the way and confuses us with undefined and void 0 being interpreted as the same thing?

[kzc]

Exactly. This has happened in the past. parse.js and output.js do some transforms of their own. Sometimes the form you see in ascii is not the same as you'd expect in the AST. It's a clever trick using the same parse/output code to check itself, but sometimes it can lead to problems. For example, string literals with "different" 'quote' characters can lead to the same AST. As can quoted and unquoted property names with various quoting styles. You may think you have a successful test when in fact you do not. expect_exact removes any doubt.

[alexlamsl]

@kzc thanks for the explanation 👍

Is there anything else that needs testing and/or changing? If not, I shall remove [wip] in the title.

[kzc]

Please remove the superfluous inner function so that undefined and var n are in the same scope.

[kzc]

Please remove the superfluous inner function so that undefined and var n are in the same scope.

[kzc]

Both tests should look like this:

        function f(undefined) {
                var n = 1;
                return typeof n == "undefined";
        }

[alexlamsl]

Updated both tests.

[kzc]

LGTM