什么意味着 then() 在递归承诺中返回

Question

我有以下代码：

function someAsyncOperation(){
    const myArray = [1,2,3,4,5];
    return Promise.resolve(myArray);
    // return Promise.reject("Reject from someAsyncOperation");
}

const random = () => {
    return Math.floor(Math.random() * 10) + 1;
}

const myFunction = (item) => {
    return someAsyncOperation() // this function returns an array
    .then((array) => {
        if(!array.includes(item)){
            console.log(`The element ${item} has NOT been found inside the array`);
            return myFunction(random()).then((r) => console.log(`Hello World ${r}`));
        }
        else{
            console.log(`The element ${item} has been found inside the array`);
            return item;
        }
    });
}

myFunction(random()).then(res => {
    // success
    console.log("Success", res);
}).catch(err => {
    // handle error here which could be either your custom error
    // or an error from someAsyncOperation()
    console.log("Error", err);
});

以下是它的结果的一些示例：

第一个答案示例

The element 10 has NOT been found inside the array
The element 8 has NOT been found inside the array
The element 7 has NOT been found inside the array
The element 5 has been found inside the array
Hello World 5
Hello World undefined
Hello World undefined
Success undefined

第二个答案示例

The element 9 has NOT been found inside the array
Nuevo elemento random generado 10
The element 10 has NOT been found inside the array
Nuevo elemento random generado 3
The element 3 has been found inside the array
Hello World 3
Hello World undefined
Success undefined

第三个答案示例

The element 5 has been found inside the array
Success 5

---

所以，我的问题是：

为什么有时会输出Hello World undefined 和Success undefined？我的意思是：then 在return myFunction(random()).then((r) => console.log(Hello World ${r})); 中是做什么的？？？

---

编辑：

Exaclty，我希望在return r（JaromandaX 的回答如下）中不仅有找到的结果，还有历史未找到结果按它们出现的顺序排列。这是我需要的一个例子：

The element 10 has NOT been found inside the array
The element 8 has NOT been found inside the array
The element 7 has NOT been found inside the array
The element 5 has been found inside the array
Hello World 10
Hello World 8
Hello World 7
Success 5

Answer 1

代码

return myFunction(random()).then((r) => console.log(`Hello World ${r}`))

将返回一个promise，该promise解析为最后一个.then中返回的值（即你有一个promise链，解析的值是链的结果）

在这种情况下，这个值为undefined，因为console.log返回的内容

你可能想要返回一个值，所以

return myFunction(random()).then((r) => (console.log(`Hello World ${r}`), r)) 

或

return myFunction(random()).then((r) => {
    console.log(`Hello World ${r}`); 
    return r;
})

将其放入您的代码中，我们得到

function someAsyncOperation(){
    const myArray = [1,2,3,4,5];
    return Promise.resolve(myArray);
    // return Promise.reject("Reject from someAsyncOperation");
}

const random = () => {
    return Math.floor(Math.random() * 10) + 1;
}

const myFunction = (item) => {
    return someAsyncOperation() // this function returns an array
    .then((array) => {
        if(!array.includes(item)){
            console.log(`The element ${item} has NOT been found inside the array`);
            return myFunction(random()).then((r) => {
                console.log(`Hello World ${r}`);
                return r;
            });
        }
        else{
            console.log(`The element ${item} has been found inside the array`);
            return item;
        }
    });
}

myFunction(random()).then(res => {
    // success
    console.log("Success", res);
}).catch(err => {
    // handle error here which could be either your custom error
    // or an error from someAsyncOperation()
    console.log("Error", err);
});