下面的代码给出正确的结果。 但是,如果将 then(callback('sum'))从承诺 sumOfPromises 中删除,并添加到对 sumOfPromises 的调用中,例如:< em> sumOfPromises()。then(callback('sum'))),然后出现 TypeError:sumOfPromises(...)未定义。 我无法理解的 sumOfPromises 中有一些错误。
<pre>P_1 is : <span id=p_1>p_1</span></pre>
<pre>P_2 is : <span id=p_2>p_2</span></pre>
<pre>Sum is : <span id=sum>sum</span></pre>
<script>
promise_1().then(callback('p_1'));
promise_2().then(callback('p_2'));
sumOfPromises(); // sumOfPromises().then(callback('sum')); gives TypeError: sumOfPromises(...) is undefined (see below)
function callback(tag) {
const result = obj => {
let e = document.getElementById(tag);
e.innerHTML = obj;
};
return result
}
function sumOfPromises() {
const promises = [promise_1(), promise_2()];
Promise.all(promises).then(function (data) {
return sumOfData(data)
}).then(callback('sum')); // then moved to top gives TypeError: sumOfPromises(...) is undefined (see upper)
}
function sumOfData(data) {
const [p_1, p_2] = data;
console.log('sumOfData input p_1 '+p_1);
console.log('sumOfData input p_2 '+p_2);
return new Promise ( (resolve) => {
var result = p_1 + p_2;
console.log('sumOfData result '+result);
resolve(result)
});
}
function promise_1() {
return new Promise((resolve) => {
const value = Math.random();
console.log('promise_1 value '+value);
resolve(value);
})
}
function promise_2() {
return new Promise((resolve, reject) => {
const value = - Math.random();
console.log('promise_2 value '+value);
resolve(value);
})
}
</script>
答案 0 :(得分:2)
您不会在sumOfPromises
中返回任何内容,因此,自undefined
起,您就无法对其附加回调:
function sumOfPromises() {
const promises = [promise_1(), promise_2()];
return Promise.all(promises).then(function (data) { // Add 'return' here
return sumOfData(data)
}).then(callback('sum'));
}
答案 1 :(得分:1)
您需要从sumOfPromises返回promise。由于在您的代码中sumOfPromises中没有return语句,因此javaScript隐式返回undefined,因此您最终会遇到这样的调用
undefined.then(callback('sum')); // which is an error
<pre>P_1 is : <span id=p_1>p_1</span></pre>
<pre>P_2 is : <span id=p_2>p_2</span></pre>
<pre>Sum is : <span id=sum>sum</span></pre>
<script>
promise_1().then(callback('p_1'));
promise_2().then(callback('p_2'));
sumOfPromises().then(callback('sum'));
function callback(tag) {
const result = obj => {
let e = document.getElementById(tag);
e.innerHTML = obj;
};
return result
}
function sumOfPromises() {
const promises = [promise_1(), promise_2()];
return Promise.all(promises).then(function (data) {
return sumOfData(data)
})
}
function sumOfData(data) {
const [p_1, p_2] = data;
console.log('sumOfData input p_1 '+p_1);
console.log('sumOfData input p_2 '+p_2);
return new Promise ( (resolve) => {
var result = p_1 + p_2;
console.log('sumOfData result '+result);
resolve(result)
});
}
function promise_1() {
return new Promise((resolve) => {
const value = Math.random();
console.log('promise_1 value '+value);
resolve(value);
})
}
function promise_2() {
return new Promise((resolve, reject) => {
const value = - Math.random();
console.log('promise_2 value '+value);
resolve(value);
})
}
</script>