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Promise.resolve() - JavaScript | MDN

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Promise.resolve(value) 方法回傳一個以 value 判定結果的 Promise 物件。若 value 是個 thenable（例如，具有 then() 方法，則回傳的 promise 將依其結果採取其最終狀態；若 value 是 promise，則作為呼叫 Promise.resolve 之結果；其他情形都將回傳以 value 實現的 promise。

Promise.resolve(value);
Promise.resolve(promise);
Promise.resolve(thenable);

value

將被 Promise 實現的引數（argument）。可以是個 Promise 或待解決的 thenable。

以 value 或作為 value 的 promise 解決的 Promise。

靜態函式 Promise.resolve 回傳判定後的 Promise。

Promise.resolve("Success").then(
  function (value) {
    console.log(value); // "Success"
  },
  function (value) {
    // not called
  },
);

var p = Promise.resolve([1, 2, 3]);
p.then(function (v) {
  console.log(v[0]); // 1
});

var original = Promise.resolve(33);
var cast = Promise.resolve(original);
cast.then(function (value) {
  console.log("value: " + value);
});
console.log("original === cast ? " + (original === cast));

// logs, in order:
// original === cast ? true
// value: 33

由於 handler 是非同步地被調用而導致相反的紀錄順序。經由這篇文章了解 then 如何運作。

// Resolving a thenable object
var p1 = Promise.resolve({
  then: function (onFulfill, onReject) {
    onFulfill("fulfilled!");
  },
});
console.log(p1 instanceof Promise); // true, object casted to a Promise

p1.then(
  function (v) {
    console.log(v); // "fulfilled!"
  },
  function (e) {
    // not called
  },
);

// Thenable throws before callback
// Promise rejects
var thenable = {
  then: function (resolve) {
    throw new TypeError("Throwing");
    resolve("Resolving");
  },
};

var p2 = Promise.resolve(thenable);
p2.then(
  function (v) {
    // not called
  },
  function (e) {
    console.log(e); // TypeError: Throwing
  },
);

// Thenable throws after callback
// Promise resolves
var thenable = {
  then: function (resolve) {
    resolve("Resolving");
    throw new TypeError("Throwing");
  },
};

var p3 = Promise.resolve(thenable);
p3.then(
  function (v) {
    console.log(v); // "Resolving"
  },
  function (e) {
    // not called
  },
);

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