Update: check template literal in no-constant-condition (fixes #12815)

[yeonjuan]

Prerequisites checklist

• I have read the contributing guidelines.
• The team has reached consensus on the changes proposed in this pull request. If not, I understand that the evaluation process will begin with this pull request and won't be merged until the team has reached consensus.

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Fixed #12815 by add template literal checking.

• overview
  This PR will make eslint to warn when a template literal(constant expression) placed in a test condition.

if (`a`) ;  // Error - Unexpected constant condition
while(`a`); // Error - Unexpected constant condition
if (`${"foo"}`); // Error - Unexpected constant condition

Is there anything you'd like reviewers to focus on?

It needs a consensus of ESlint members.
Marked as anUpdate, as it will generate more warning, but I think it can be accepted as a bug fix.

[mdjermanovic]

I think this is certainly a correct change in behavior 👍

Just not sure how to label it (bug/enhancement/breaking).

[mdjermanovic]

It might be nice to add a test with a tagged template literal, which would be a constant if there's no tag.

[yeonjuan]

@mdjermanovic Thanks for commenting! I have a question about it. :)

which would be a constant if there's no tag.

What is the tagged template literal without a tag? Just mean template literal?

tag`template literal ${expression}`

[yeonjuan]

sorry for closing it, it was mistake.

[mdjermanovic]

Sorry for the confusion! Was thinking of a test case like the one you added now: if (tag`a`)

(tagged template with a constant template literal)

[mdjermanovic]

While at template literals, wondering should the rule also report something like this:

if (`a${b}`) {}

This is similar to the logic for typeof b: it isn't a constant, but it's an expression that always evaluates to a truthy string value and thus may be treated as a constant when inBooleanPosition is true?

[kaicataldo]

This is similar to the logic for typeof b: it isn't a constant, but it's an expression that always evaluates to a truthy string value and thus may be treated as a constant when inBooleanPosition is true?

I think this makes sense 👍

[yeonjuan]

@mdjermanovic

This is similar to the logic for typeof b: it isn't a constant, but it's an expression that always evaluates to a truthy string value and thus may be treated as a constant when inBooleanPosition is true?

I agree. thanks for commenting :). I have some questions.
If then, should we change these cases to be also reported? And should I treat those cases in this pr?
online-demo

/*eslint no-constant-condition: "error"*/
if ("a" + b) {} // no-error

[mdjermanovic]

/*eslint no-constant-condition: "error"*/
if ("a" + b) {} // no-error

I think this should be a separate issue or PR.

[yeonjuan]

@mdjermanovic
Thanks for the review. I changed it.
To note, There are other cases we should treat, which related to string concatenation generating the truthy result. (in both string literal and template literal)

if (`${"foo"}${foo}`) {};
if("foo" + foo) {};

And I think both cases can be fixed together in another PR. Thanks :)

[mdjermanovic]

LGTM, thanks!

[kaicataldo]

LGTM, thanks!