The spec draft text of all methods looks incorrect

[zloirock]

Since steps 3.b and 3.c in the loop and they work with existent entry, in case the first map entry key is not something that we search, the value of this entry will be replaced to passed.

Something like

let map = new Map([['a', 1]]);
// map.getOrInsert('b', 2):
(() => {
  for (let [key, value] of map) {
    if (key === 'b') return value;
    map.set(key, 2);
    return 2;
  }
})();
console.log(map); // => Map { 'a': 2 }

[dminor]

Yes, that was my mistake. We have https://github.com/tc39/proposal-upsert/pull/61 open to fix this up, if you have some time, please have a quick look and see if this addresses your concerns.

[zloirock]

Sorry, I missed this PR. It seems it's fixed here.