[英]Calling a promise function recursively
I'm trying to call a promise function recursively. 我试图递归调用一个promise函数。
The following call service.getSentenceFragment() returns upto 5 letters from a sentence ie 'hello' from 'helloworld. 以下调用service.getSentenceFragment()从一个句子(即“ helloworld”中的“ hello”)最多返回5个字母。 Providing a nextToken value as a parameter to the call returns the next 5 letters in the sequence.
提供nextToken值作为调用参数将返回序列中的下5个字母。 ie 'world'.
即“世界”。 The following code returns 'hellohelloworldworld' and does not log to the console.
以下代码返回“ hellohelloworldworld”,并且不登录到控制台。
var sentence = '';
getSentence().then(function (data)) {
console.log(sentence);
});
function getSentence(nextToken) {
return new Promise((resolve, reject) => {
getSentenceFragment(nextToken).then(function(data) {
sentence += data.fragment;
if (data.nextToken != null && data.nextToken != 'undefined') {
getSentence(data.NextToken);
} else {
resolve();
}
}).catch(function (reason) {
reject(reason);
});
});
}
function getSentenceFragment(nextToken) {
return new Promise((resolve, reject) => {
service.getSentenceFragment({ NextToken: nextToken }, function (error, data) {
if (data) {
if (data.length !== 0) {
resolve(data);
}
} else {
reject(error);
}
});
});
}
Cause when you do this: 原因在您执行此操作时:
getSentence(data.NextToken);
A new Promise chain is started, and thecurrent chain stays pending forever. 新的Promise链已启动,而当前链将永远挂起。 So may do:
因此可以这样做:
getSentence(data.NextToken).then(resolve, reject)
... but actually you could beautify the whole thing to: ...但是实际上您可以将整件事美化为:
async function getSentence(){
let sentence = "", token;
do {
const partial = await getSentenceFragment(token);
sentence += partial.fragment;
token = partial.NextToken;
} while(token)
return sentence;
}
And watch out for this trap in getSentenceFragment
- if data
is truthy but data.length
is 0, your code reaches a dead end and the Promise will timeout 并注意
getSentenceFragment
陷阱-如果data
是真实的,但data.length
为0,则您的代码将达到死胡同,并且Promise将超时
// from your original getSentenceFragment...
if (data) {
if (data.length !== 0) {
resolve(data);
}
/* implicit else: dead end */
// else { return undefined }
} else {
reject(error);
}
Instead, combine the two if
statements using &&
, now our Promise will always resolve or reject 相反,使用
&&
组合两个if
语句,现在我们的Promise将始终解析或拒绝
// all fixed!
if (data && data.length > 0)
resolve(data);
else
reject(error);
You could recursively call a promise like so: 您可以像这样递归地调用一个Promise:
getSentence("what is your first token?")
.then(function (data) {
console.log(data);
});
function getSentence(nextToken) {
const recur = (nextToken,total) => //no return because there is no {} block so auto returns
getSentenceFragment(nextToken)
.then(
data => {
if (data.nextToken != null && data.nextToken != 'undefined') {
return recur(data.NextToken,total + data.fragment);
} else {
return total + data.fragment;
}
});//no catch, just let it go to the caller
return recur(nextToken,"");
}
function getSentenceFragment(nextToken) {
return new Promise((resolve, reject) => {
service.getSentenceFragment({ NextToken: nextToken }, function (error, data) {
if (data) {
if (data.length !== 0) {
resolve(data);
}
} else {
reject(error);
}
});
});
}
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