如果多个 javascript Promise 使用 Promise.allSettled() 解决，Catch() 不会处理对 Promise 的拒绝

Question

I can see a strange behaviour when all of the request is being failed for multiple javascript Promises to settle with Promise.allSettled, .catch is unable to handle rejection.

const API_URL = "https://jsonplaceholder.typicode.com/";

const spinner = document.getElementById("spinner");
const output = document.getElementById("output");

function queryApi(endpoint){
    return fetch(API_URL + endpoint).then(response => {
      return response.ok ? response.json() : Promise.reject("Unsuccessful response");
  })
}

const promise = Promise.allSettled([
queryApi("_posts"),
queryApi("_comments"),
queryApi("_users")
]);

promise
.then(results => {
console.log(results);
const posts = results[0];
const comments = results[1];
const users = results[2];
const statistics = [];
if(posts.status === 'fulfilled'){
  statistics.push(`${posts.value.length} posts`);
}
if(comments.status === 'fulfilled'){
  statistics.push(`${comments.value.length} comments`);
}
if(users.status === 'fulfilled'){
  statistics.push(`${users.value.length} users`);
}
output.innerText = statistics.join("\n");
})
.catch(error => {
  console.warn(error);
  output.innerText = ":(";
})
.finally(() => {
  spinner.remove();
});

Answer 1

Promise.allSettled() always resolves, even if some promises passed to it reject. It resolves with an array of outcomes for each promise.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/allSettled

You can use Promise.all() instead, which will resolve when all passed promises resolve or reject when any passed promise rejects.

Answer 2

The catch statement will be executed when the error occurs. As the Promise.allSettled() always resolves, you can manually throw an error ie

 if(posts.status === "rejected"){ throw new Error('a status was rejected'); }