Use .toString() as functions' representation, fix #5140

[OpportunityLiu]

Use .toString() as functions' representation.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #5140

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

We should check if the value is a class here and use value.name if so.

[OpportunityLiu]

To check if a value is a class, toString() is still needed to be called.

I think there is no need to do so since the returned value is proceeded by a hash function immediately.

[OpportunityLiu]

https://stackoverflow.com/questions/30758961/how-to-check-if-a-variable-is-an-es6-class-declaration

[kamilmysliwiec]

What I'm trying to say is that it would be great if we could still return value.name for classes, not .toString().

[OpportunityLiu]

I mean,... How?

[kamilmysliwiec]

We can very likely limit the number of characters (slice) to slightly optimize this process. ~50 should be enough to distinguish arrow functions