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
js
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
js
Promise.resolve("Success").then(
function (value) {
console.log(value); // "Success"
},
function (value) {
// not called
},
);
判定陣列
js
var p = Promise.resolve([1, 2, 3]);
p.then(function (v) {
console.log(v[0]); // 1
});
判定另一個 Promise
js
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。
判定 thenable 及拋出 Error
js
// 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
},
);
規範
Specification |
---|
ECMAScript Language Specification # sec-promise.resolve |
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