js-is-weird.js

// I learned in this post (https://overreacted.io/react-as-a-ui-runtime/)
// that arguments passed to a function run before the function itself:
/**
 * // This runs second
 * outerFunction(
 *  // This runs first
 *  innerFunction()
 * )
 */
// This makes sense. But I wondered what it looked like in practice.
// I'm probably not testing it the right way, but I found something interesting.

const obj = {
  fn: (arg) => {
    // fn() is basically just an identity function that returns its own argument
    return arg
  }
}

// Store the function so we can reuse it later
const fn = obj.fn

// These three results make sense
obj.fn(1 + 1) // => 2
obj.fn(obj.foo = 'bar') // => 'bar'
obj.fn(obj.fn = () => 1) // => () => 1

// This is where it gets weird.
// `delete obj.fn` doesn't appear to run first, since `1` is returned,
// which is the return value of the previous function.
obj.fn(delete obj.fn) // => 1

// Let's put the original function back in place
obj.fn = fn

obj.fn(delete obj.fn) // => true

obj.fn(obj.fn = () => 1) // error! obj.fn is not a function