feat(eslint-plugin): add new rule no-empty-function

[scottohara]

Extends the no-empty-function rule from ESlint core to allow for empty constructor bodies when parameter properties are used (a.k.a. concise constructor functions).

Fixes #426

Examples of allowed code

/*eslint @typescript-eslint/no-empty-function: "error"*/

class Person {
  constructor(private firstName: string, private surname: string) {}
}

...which is functionally equivalent to writing:

class Person {
  private firstName: string;
  private surname: string;

  constructor(firstName: string, surname: string) {
    this.firstName = firstName;
    this.surname = surname;
  }
}

Aside from this one specific case, the rule passes through to ESLint core no-empty-function for everything else, so the following still applies:

Examples of valid code

/*eslint @typescript-eslint/no-empty-function: ["error", { allow: ['constructors'] }]*/

class Person {
  constructor(firstName: string, surname: string) {}
}

/*eslint @typescript-eslint/no-empty-function: ["error", { allow: ['methods'] }]*/

class Person {
  otherMethod(firstName: string, surname: string) {}
}

Examples of invalid code

/*eslint @typescript-eslint/no-empty-function: "error"*/

class Person {
  constructor(firstName: string, surname: string) {}
}

/*eslint @typescript-eslint/no-empty-function: "error"*/

class Person {
  otherMethod(firstName: string, surname: string) {}
}

[JamesHenry]

Definitely one of those awkward ones that is not super clear-cut, but overall seems like a reasonable change and I think matches user expectations

[codecov]

Codecov Report

Merging #626 into master will decrease coverage by 0.02%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
- Coverage   94.27%   94.25%   -0.03%     
==========================================
  Files         106      107       +1     
  Lines        4369     4388      +19     
  Branches     1202     1208       +6     
==========================================
+ Hits         4119     4136      +17     
- Misses        145      147       +2     
  Partials      105      105

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/index.ts	100% <100%> (ø)	⬆️
...kages/eslint-plugin/src/rules/no-empty-function.ts	88.88% <88.88%> (ø)

[bradzacher]

Code LGTM! Thanks for doing this!

[joaovieira]

Just merged an older eslint ruleset (used in JS projects) with a plugin:@typescript-eslint/recommended one and started getting no-useless-constructor and no-empty-function errors as they were enabled in the old ruleset. Just seen both have TS rules now, which is great! ❤️

Do you think these could be added to the recommended ruleset?

[bradzacher]

We only update recommended rulesets on breaking changes.
We will discuss the changes to the set when it comes time for a breaking change.

[onichandame]

Thx for the fix. But how can I use this PR? Any rules that I can use to override the recommended ruleset?

[bradzacher]

please read the rule docs:
https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-empty-function.md