轉發承諾導致Promise鏈 - 爲什麼我得到Promise對象而不是被拒絕的值

Question

我在理解轉發承諾結果（嵌套承諾）如何工作時遇到問題。

此代碼的工作，因爲我預計（在最終然後我得到的整數值）：

function opThatResolves() { 
 
    return Promise.resolve(1); 
 
} 
 

 
function opThatWillBeForwarded(x) { 
 
    return new Promise(function(resolve, reject) { 
 
    resolve(yetAnotherNestedPromise(x)); 
 
    }) 
 
} 
 

 
function yetAnotherNestedPromise(x) { 
 
    return Promise.resolve(-x); 
 
} 
 

 
opThatResolves() 
 
    .then(x => opThatWillBeForwarded(x)) 
 
    .then(x => x * 2) 
 
    .then(x => x * 2) 
 
    .then(x => console.log("Resolved: " + x)) 
 
    .catch(x => console.log("Rejected: " + x))

所以我想，如果我改變resolve到reject我會得到類似的結果（整數值，但沒有雙倍* 2乘法）在catch塊中。不過，我得到一個完整的Promise對象：

function opThatResolves() { 
 
    return Promise.resolve(1); 
 
} 
 

 
function opThatWillBeForwarded(x) { 
 
    return new Promise(function(resolve, reject) { 
 
    reject(yetAnotherNestedPromise(x)); 
 
    }) 
 
} 
 

 
function yetAnotherNestedPromise(x) { 
 
    return Promise.resolve(-x); 
 
} 
 

 
opThatResolves() 
 
    .then(x => opThatWillBeForwarded(x)) 
 
    .then(x => x * 2) 
 
    .then(x => x * 2) 
 
    .then(x => console.log("Resolved: " + x)) 
 
    .catch(x => console.log("Rejected: " + x))

爲什麼reject不 「解包」 無極從yetAnotherNestedPromise就像resolve沒有回來？

Answer 1

因爲它不應該首先收到Promise。

reject回調應該包含原因失敗（即Error）。

function opThatResolves() { 
    return Promise.resolve(1); 
} 

function opThatWillBeForwarded(x) { 
    return new Promise(function(resolve, reject) { 
    reject(new Error("failed")); 
    }) 
} 

opThatResolves() 
    .then(x => opThatWillBeForwarded(x)) 
    .then(x => x * 2) 
    .then(x => x * 2) 
    .then(x => console.log("Resolved: " + x)) 
    .catch(x => console.log("Rejected: " + x)); 

注意，錯誤可以通過後續的錯誤回調重新拋出：

new Promise((resolve, reject) => { 
    reject(new Error("failed !")); 
}) 
    .then(null, reason => { 
     console.log(1, reason); 
     throw reason; 
    }) 
    .catch(reason => { 
     console.log(2, reason); 
    }); 

還要注意的是，如果一個錯誤回調未能再次引發錯誤（或拋出另一個錯誤），隨後then的方法將推出成功回調：

new Promise((resolve, reject) => { 
    reject(new Error("failed !")); 
}) 
    .then(null, reason => { 
     console.log(1, reason); 
    }) 
    .then(result => { 
     console.log("Success! Let's throw something else..."); 
     throw new Error("Another failure"); 
    }) 
    .catch(reason => { 
     console.log(2, reason); 
    });