Promise.resolve()

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。

語法

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

參數

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

回傳值

以 value 或作為 value 的 promise 解決的 Promise。

描述

靜態函式 Promise.resolve 回傳判定後的 Promise。

範例

使用 Promise.resolve 靜態方法

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

判定陣列

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

判定另一個 Promise

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 handlers 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。

判定 thenables 及拋出 Errors

// 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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