Add callable-types rule

[ghost]

PR checklist

• Addresses an existing issue: New rules for linting type declarations #1844
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

Added the callable-types rule, which recommends to use a function type over an interface or type literal with just a call signature.

[nchen63]

@andy-ms thanks!

[PsySecCorp]

I've found having a module with common interfaces for callbacks improves code readability. For example:

export interface IError {
  (
    message: string,
    data: any,
  ): void;
}

export interface IDone {
  (
    error: any,
    data: any,
  ): void;
}

export interface INext {
  (
    error: any,
    data: any,
  ): void;
}

export interface IPassthru {
  (
    data: any,
  ): void;
}

This rule appears to prevent this. I'll be turning this rule off in my local tslint.json, but I'm curious what problem this rule is solving. Thanks!

[ghost]

You can just as easily use a type alias.

export type Error = (message: string, data: any) => void;

Also, in your example, if you used a specific function type instead of using a common one, you could avoid using any. You could also use a generic: export type Error<T> = (message: string, data: T) => void.

[PsySecCorp]

Oh my. Most helpful, @andy-ms Thanks for the breakdown!