Fix for IE11 check before `preserveToString`

[AlastairTaft]

When in IE11 the 'preserve string' method crashes on startsWith. startsWith doesn't exist in IE11 so the preserveToString method doesn't work (there's nothing to preserve).

Added a check to prevent it running if it doesn't need to and it's going to cause an error.

[ljharb]

Thanks for the report!

I'd rather fix this in the place overrideNative is being called on a non-present native. It seems like if this checked that startsWith and endsWith both exist, before entering the block, it might solve it also?

That said, which version of IE 11 are you using? In my copy, this does not error out.

[AlastairTaft]

In the overrideNative method you might be relying on the bit of code defineProperty(object, property, replacement, true); to set and make the startsWith property exist, but I'm not sure with this.

I think the startsWithIsCompliant is being set to undefined because it looks like both methods startsWithRejectsRegex and startsWithHandlesInfinity are probably returning undefined because they have checks like this at the start return String.prototype.startsWith && .... So the setting of the value will be startsWithRejectsRegex() && startsWithHandlesInfinity will be undefined && undefined.

My IE version says Version: 11.0.9600.17937 UpdateVersion:11.0.22

[ljharb]

I think you're totally correct that I'm incorrectly calling overrideNative in this case - I'm just surprised it hasn't failed before.

If you're open to editing and rebasing your PR so that it avoids https://github.com/paulmillr/es6-shim/blob/master/es6-shim.js#L735-L739 entirely when startsWith or endsWith aren't present, I'd be happy to merge that! As is, I'd prefer overrideNative to blow up when passed a nonexistent native, since it indicates that I did something wrong :-)

[ljharb]

Awesome! If you can fix these comments, and then rebase it down to a single commit on top of the latest master, this will be good to go!

[ljharb]

Please be sure to rebase this - merge commits make git logs sad

[AlastairTaft]

My git skills are definitely lacking because I can't seem to squash these commits.

[ljharb]

@AlastairTaft git fetch ; git rebase -i origin/master and edit the messages according to the instructions, and when you're done, git push -f

[ljharb]

@AlastairTaft Thanks for reporting this! I fixed this myself, since it's pretty important and I couldn't wait any longer for your rebase. I appreciate the effort!

[AlastairTaft]

Sorry, this dropped off my radar.