constant-folding: Fix some literal types used in .join(), closes #612

[goto-bus-stop]

The .join() inlining function was checking for isLiteral() on each array element, and then using the element's .value. Some literals don't have .values though: null literals, regexp literals and template literals.

This patch fixes .join()ing regexp literals and bails out of joining template literals entirely.

Null literals already did the right thing despite not having a .value, because .join() appears to treat undefined and null the same.

[goto-bus-stop]

Lint is failing, but if I npm run fix some files that are not relevant to this PR get changed too. Should I commit all of that or just the fixes to the files I changed?

e; I fixed just the lines i changed, looks like that did it ✌️

[j-f1]

@goto-bus-stop What about people using template strings like `foo`? You could check if templateLiteral.expressions.length === 0, which would indicate that it’s an ordinary template literal.

[goto-bus-stop]

Ah yes, that's quite simple. Thanks for the suggestion!

[vigneshshanmugam]

IMO, we can do path.evaluate(el) and deopt.

If its gonna be killing performance which in most cases should not be a problem since the path would already be cached during the traversal phase.. Thoughts?

[goto-bus-stop]

You mean doing evaluate instead of all the is* checks?

e; The only problem I can see is that there's no access to the path in these replacement functions, so we'd need to pass it in somehow…

[vigneshshanmugam]

check this https://github.com/babel/babel/blob/7.0/packages/babel-traverse/src/path/evaluation.js#L391

[boopathi]

Thanks! Looks good to me. Can you address the pending comments?

[goto-bus-stop]

Added a commit to use path.evaluate(). Previously the NodePath was not available inside replacement functions, only the node itself. I changed it to pass the NodePath to replacement functions, and changed all existing replacement functions to use this.node instead of just this where necessary. It makes the diff quite noisy.

[boopathi]

LGTM. Looks like formatting issues on CI. you can fix it with yarn format

[goto-bus-stop]

Oops, should've checked lint before committing. Thanks for the heads-up