Adds strictMode option to strict-mode transform

[jamiebuilds]

This allows you to disable "use strict"; when using the commonjs module transform.

[codecov-io]

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[loganfsmyth]

Looks good.

[jmm]

What's the use case for this?

What does "turn strict off" mean -- pass strict: false to transform-strict-mode?

[loganfsmyth]

@jmm The issue is we have this plugin used inside transform-es2015-modules-commonjs via inherit, and that plugin also has an option called strict, so you can't toggle one but not the other, unexpectedly.

[jmm]

Thanks @loganfsmyth. Sorry, what I meant is, what is the use case for disabling Use Strict when transforming modules at all? When is it a good idea?

I was also wondering what's the purpose of the undocumented strict option to transform-es2015-modules-commonjs. I took a look at the source and couldn't totally figure it out yet. It looks like it controls at least a couple of other things besides adding Use Strict, one of which I didn't grok right away. The other is adding an __esModule export and I'm not sure what that has to do with adding Use Strict, and I see that you get 3 different transformations for strict values of true|false|undefined.

It's still not clear to me what the docs mean by:

turn strict off

[loganfsmyth]

It's not a good idea, but we supported it in Babel 5 and James was transitioning a project from Babel 5 to Babel 6 and was hitting this as an issue because he couldn't toggle one flag without the other.

The other strict isn't related to strict mode, it's related to strictly processing modules, e.g. not adding another magic export called __esModule to the file that isn't actually declared in the file. I don't know why it was added or if it is used by anyone though.

[jmm]

Thanks Logan.

I'm guessing that was discussed on Slack...in any case, in general, if things are discussed on Slack it would be super helpful if people could record the background information and reasoning for decisions from there in PRs / issues.

It's not a good idea

We should make that clear in the README. I was helping a module author on the forum who used a preset that disables the Use Strict directive in the module transform to accommodate a user who wanted to concatenate their module with other code. Well, actually that user's package contains a script not a module, so omitting the module transform would be the appropriate solution there -- but it just goes to show that there's a lot of misunderstanding around strict mode and modules and we shouldn't make it sound like a good idea.

The other strict isn't related to strict mode, it's related to strictly processing modules, e.g. not adding another magic export called __esModule to the file that isn't actually declared in the file.

Thanks for that info. It does affect output of Use Strict directive though. You can get three different versions of output:

strict: true
=> Use Strict: yes, __esModule: no

strict: false
=> Use Strict: no, __esModule: yes

strict: undefined
=> Use Strict: yes, __esModule: yes

I don't know why it was added or if it is used by anyone though.

We should document or deprecate it. The preset that I mentioned above that came up on the discussion forum is using it for the purpose of omitting the Use Strict directive.