Promise recursion

PromiseRecursion.js

function fetchPages(resultsArray, page, end) {
  return new Promise((resolve, reject) => {
    fetch(`https://my-api.com/?page=${page}`)
      .then((res) => {
        // res.json
        // {
        //   data: [1,2,3],
        //   nextPage: 1,
        //   previousPage: 0,
        // }
        const { data, nextPage } = res;
        console.log(nextPage);
        const thisPageResults = resultsArray.concat(data);
        if (nextPage !== null && nextPage < end) {
          // doesn't work, it will call correctly the function (that returns a promise) 
          // and fetch the data but won't trigger the last resolve(thisPageResults)
          return fetchPages(thisPageResults, nextPage, end);
          // works - why?
          return resolve(fetchPages(thisPageResults, nextPage, end));
        }
        return resolve(thisPageResults);
      })
      .catch(err => reject(err));
  });
}

const resultsArray = [];
fetchPages(resultsArray, 0, 3)
  .then(results => console.log(results))
  .catch(err => console.log(err));

If you don't call resolve the promise won't return anything.
Every time you're calling fetchPages you're creating a new Promise and you're calling resolve only on the last Promise you create.
By calling resolve(fetchPages...) you're calling resolve on all the promises from the last one to the first one.

That said, I'd rather not use new Promise: https://gist.github.com/framp/8e74dfa713049be1ca6b70e8a3557659