invoke-as-method.md

Useful to call-this proposal or Extensions proposal

The call-this or Extensions propose the syntax to invoke a function as method form, they could utilize runtime semantic of the functions with this parameter to mitgate the confusion of free methods and protect the users. For example:

// zip-method.js
export function zip (this, otherArray) {
  return this.map( (a, i) => [a, otherArray[i]] )
}

// call-this example

import {zip} from "./zip-method.js"

[1, 2]
  .map(x => x * 10)
  ~>zip([1, 2]) // call-this syntax
  .flat()
//=> [10, 1, 20, 2]

zip([10, 20], [1, 2]) // throw TypeError, so avoid misuse

Extensions proposal support similar pattern, already protect the users without this parameter functions

// Extensions example

// import zip as an extension method
import ::{zip} from "./zip-method.js" 

[1, 2]
  .map(x => x * 10)
  ::zip([1, 2]) // invoke extension method
  .flat()
//=> [10, 1, 20, 2]

zip // throw ReferenceError

import {zip} from "./zip-method.js"
[10, 20]::zip([1, 2]) // ReferenceError

But Extensions proposal could be revised to only accept functions with this parameter, provide even stronger protection.

// zip-util.js
// an util function, no `this` parameter
export function zip (array, otherArray) {
  return array.map( (a, i) => [a, otherArray[i]] )
}

import ::{zip} from "./zip-util.js" // TypeError