Javascript puzzle - seal the secret

Really nice exercise! Got me thinking. Simplest approach would be to introduce state to the function, i.e. a member variable (this.strings = []), but that wouldn't seal the strings away. A quick inspection of the code via .toString() would reveal the name of the member and thus reveal the strings.

I went on to more of a scripting approach by introducing a factory function that would create a seal function that would then store away the strings, never to be accessed outside of the function itself:

const sealFactory = (sealedStrings = []) => {
   return (string) => {
     if (sealedStrings.includes(string)) {
       return true
     }

     sealedStrings.push(string)
     seal = sealFactory(sealedStrings)

     return false
   }
}

let seal = sealFactory()

console.log(seal("Foo")) // false
console.log(seal("Bar")) // false
console.log(seal("Foo")) // true
console.log(seal("Bar")) // true

I didn't really find a quick way to put this all in a single function that doesn't return a function via side effect, so I kinda violated the "externals" and "self-contained" constraints, but at least the strings are safe!

Using an IIFE should do the trick

const seal = ((sealedStrings = []) => {
  let sealedString;
  return (str) => {
    if (sealedString === str) {
      return true;
    }
    if (sealedString === undefined) {
      sealedString = str;
      return 'saved';
    }
    return false;
  };
})();

console.log(seal("Foo")); // saved
console.log(seal("Bar")); // false
console.log(seal("Foo")); // true
console.log(seal("Bar")); // false

`

I used an empty array to seal more than one string. The factory function could take an empty array initially, but I thought I'd add it as the default value. For some reason I thought multi string support was an unclearly communicated requirement, but it doesn't really change much.