What does this arrow function JavaScript code mean?

Question

I am trying to understand HomeAssistant frontend source code. I have found function definition that I dont uderstand well. I dont understand this syntax (model.entity is a string)...

export function createHasDataGetter(model) {
  return [
    ['restApiCache', model.entity],
    entityMap => !!entityMap,
  ];
}

It looks like smth like:

return [[string, string], bool]?

What is exacly teturn type of this function? Is this just bool? If yes, does it mean entityMap is string array?

If you know it's an arrow function already, what are you confused about? Seems like you already know it's a function and not a `bool`. — , Aug 03 '16 at 14:52
Duplicate of: http://stackoverflow.com/questions/24900875/whats-the-meaning-of-an-arrow-formed-from-equals-greater-than-in-javas?noredirect=1&lq=1 — Paul, Aug 03 '16 at 14:56
`entityMap => !!entityMap` is equivalent to `function ( entityMap ) { return !! entityMap; }` — Paul, Aug 03 '16 at 14:56
Ok, so what actually is createHasDataGetter returning? Is it bool? Returned by arrow function? Why return statement has 2 argumets (array of strings and function returning bool) — Marcin Kurpiel, Aug 03 '16 at 15:07

score 3 · Answer 1 · edited May 23 '17 at 11:51

See "Truthy" on MDN:

In JavaScript, a truthy value is a value that translates to true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN).

entityMap => !!entityMap maps entityMap to a canonical boolean value, true or false. See also What is "!!" in C?.

If entityMap has a truthy value, then !entityMap is false, and !!entityMap is true.

If entityMap has a falsy value, then !entityMap is true and !!entityMap is false.

What does this arrow function JavaScript code mean?

1 Answers1