Add prefer-function-over-method rule

[andy-hanson]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update
• Enable CircleCI for your fork (https://circleci.com/add-projects)

What changes did you make?

Added the no-unused-this rule, which warns for methods that don't use this (they could be functions).

Also, once microsoft/TypeScript#13217 is in, I'll add an exception for override, since that's a legitimate case where you would need to write a method that might not use this.

[ajafff]

Just a suggestion:
You could detect methods that only calls itself recursive.

class Foo {
    bar() { this.bar(); }
    ~~~ [lorem ipsum]
}

If you want to go the easy way, you could "borrow" my implementation for this rule: https://github.com/ajafff/tslint-consistent-codestyle/blob/master/rules/preferStaticMethodRule.ts

[andy-hanson]

@ajafff Thanks! Have you considered turning some of your rules into pull requests here?
no-collapsible-if and no-unnecessary-else were some I was considering working on, but won't have to, it seems.

[ajafff]

On second thought the name of this rule is a bit misleading.
For me no-unused-this would be something like the following:

// function is not using this, but declares it in parameter list
function doStuff(this: Foo, bar: Bar) {
                 ~~~~~~~~~ [unused this]
    return bar;
}

[nchen63]

name is misleading since it does allow this. I don't have a good name, but what about something like prefer-functions-over-methods

[andy-hanson]

How about method-must-use-this?

[nchen63]

That sort of makes it sound like the rule is suggesting that this should be added to the method.

[nchen63]

@andy-hanson thanks!