@typescript-eslint/prefer-function-type

An interface with a call signature and nothing else describes a function. type Handler = (event: Event) => void says the same thing in one line.

Inline object types and new (...) signatures are checked too. An interface that extends another interface is left alone.

--fix rewrites the interface to a type alias, and the object type to a function type.

Configure#

Off by default. To turn it on:

[lint.rules]
"@typescript-eslint/prefer-function-type" = "error" # or "warn"

Examples#

Reported:

interface Handler {
  (event: Event): void;
}

function listen(callback: { (event: Event): void }) {}

Not reported:

type Handler = (event: Event) => void;

function listen(callback: (event: Event) => void) {}

A type with another member, or with more than one call signature, can't be written as a function type:

interface Logger {
  (message: string): void;
  level: number;
}

interface Parse {
  (text: string): object;
  (text: string, strict: boolean): object;
}

interface Timed extends Logger {
  (message: string): void;
}

In an interface, this as a type means the interface itself. A function type has no such this, so the rule points at it and doesn't offer a fix:

interface Chain {
  (step: string): this;
}