我已经实现了一些代码，其中异步代码后跟一些同步函数。例如：
function processSomeAsyncData() {
asyncFuncCall()
.then(syncFunction)
.catch(error);
}
如果我理解正确then也是承诺。那么，我应该在同步代码中创建一个承诺吗？
function syncFunction() {
const p = new Promise (function (resolve, reject) {
//Do some sync stuff
...
resolve(data);
}
return p;
}
如果没有必要，如果发生错误，如何拒绝来自同步代码的承诺？
答案 0 :(得分：6)
您无需明确创建新承诺。有一种更简单的方法。
这个例子是设计的，因为它永远不会失败，但重点是你不必创建一个承诺，你不必返回一个解决方案（val）。
function syncFunction() {
var j = "hi"
if(j){
return j;
}
return new Error('i am an error');
}
这将有效：
asyncFunction()
.then(syncFunction);
但如果你反过来这样做了：
syncFunction()
.then(asyncFunction);
您必须将syncFunction定义为：
function syncFunction() {
var j = "hi"
return new Promise((resolve, reject) => {
if(j){
return resolve(j);
}
return reject('error');
})
}
编辑：要向所有非信徒证明，请在电脑上给这个家伙一个机会。证明您有这么多选项可供您使用。 ：）
var Promise = require('bluebird');
function b(h) {
if(h){
return h;
}
return Promise.resolve('hello from b');
}
function a(z) {
return new Promise((resolve, reject)=> {
if(z){return resolve(z)};
return resolve('hello from a');
})
}
a().then(b).then(x => console.log(x)).catch(e => console.log(e));
b().then(a).then(x => console.log(x)).catch(e => console.log(e));
答案 1 :(得分：3)
这是可选的。
如果您从syncFunction返回承诺，那么您的原始承诺将在新承诺结算后解决，并且新承诺返回的任何值将传递给链中的下一个then
如果您返回非Promise值，那么该值将传递给链中的下一个then。
要从syncFunction中拒绝，只需抛出异常。
答案 2 :(得分：2)
没有。可以从同步代码调用同步函数，并且应该始终同步失败！它们不需要以任何方式符合异步调用者。如果发生错误，则抛出错误。试试吧：
. sysuse auto, clear
(1978 Automobile Data)
. saveold auto, version(12) replace
(saving in Stata 12 format, which can be read by Stata 11 or 12)
file auto.dta saved
.
. rsource, terminator(XXX)
Assumed R program path: "/usr/local/bin/R"
Beginning of R output
> library("foreign")
> mydata<-read.dta("~/Desktop/auto.dta")
> mydata$rep78 <- relevel(as.factor(mydata$rep78), ref = 4)
> m1<-lm(price ~ rep78,data = mydata)
> summary(m1)
Call:
lm(formula = price ~ rep78, data = mydata)
Residuals:
Min 1Q Median 3Q Max
-3138.2 -1925.2 -1181.5 369.5 9476.8
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 6071.5 702.4 8.643 2.38e-12 ***
rep781 -1507.0 2221.3 -0.678 0.500
rep782 -103.9 1266.4 -0.082 0.935
rep783 357.7 888.5 0.403 0.689
rep785 -158.5 1140.6 -0.139 0.890
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 2980 on 64 degrees of freedom
(5 observations deleted due to missingness)
Multiple R-squared: 0.01449, Adjusted R-squared: -0.0471
F-statistic: 0.2353 on 4 and 64 DF, p-value: 0.9174
>
End of R output
.
. /* Old Way */
. char rep78[omit]4
. xi: reg price i.rep78
i.rep78 _Irep78_1-5 (naturally coded; _Irep78_4 omitted)
Source | SS df MS Number of obs = 69
-------------+---------------------------------- F(4, 64) = 0.24
Model | 8360542.63 4 2090135.66 Prob > F = 0.9174
Residual | 568436416 64 8881819 R-squared = 0.0145
-------------+---------------------------------- Adj R-squared = -0.0471
Total | 576796959 68 8482308.22 Root MSE = 2980.2
------------------------------------------------------------------------------
price | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
_Irep78_1 | -1507 2221.338 -0.68 0.500 -5944.633 2930.633
_Irep78_2 | -103.875 1266.358 -0.08 0.935 -2633.715 2425.965
_Irep78_3 | 357.7333 888.5353 0.40 0.689 -1417.32 2132.787
_Irep78_5 | -158.5 1140.558 -0.14 0.890 -2437.026 2120.026
_cons | 6071.5 702.4489 8.64 0.000 4668.197 7474.803
------------------------------------------------------------------------------
.
. /* Post-Stata 11 Way */
. reg price ib4.rep78
Source | SS df MS Number of obs = 69
-------------+---------------------------------- F(4, 64) = 0.24
Model | 8360542.63 4 2090135.66 Prob > F = 0.9174
Residual | 568436416 64 8881819 R-squared = 0.0145
-------------+---------------------------------- Adj R-squared = -0.0471
Total | 576796959 68 8482308.22 Root MSE = 2980.2
------------------------------------------------------------------------------
price | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
rep78 |
1 | -1507 2221.338 -0.68 0.500 -5944.633 2930.633
2 | -103.875 1266.358 -0.08 0.935 -2633.715 2425.965
3 | 357.7333 888.5353 0.40 0.689 -1417.32 2132.787
5 | -158.5 1140.558 -0.14 0.890 -2437.026 2120.026
|
_cons | 6071.5 702.4489 8.64 0.000 4668.197 7474.803
------------------------------------------------------------------------------
. fvset base 4 rep78
. reg price i.rep78
Source | SS df MS Number of obs = 69
-------------+---------------------------------- F(4, 64) = 0.24
Model | 8360542.63 4 2090135.66 Prob > F = 0.9174
Residual | 568436416 64 8881819 R-squared = 0.0145
-------------+---------------------------------- Adj R-squared = -0.0471
Total | 576796959 68 8482308.22 Root MSE = 2980.2
------------------------------------------------------------------------------
price | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
rep78 |
1 | -1507 2221.338 -0.68 0.500 -5944.633 2930.633
2 | -103.875 1266.358 -0.08 0.935 -2633.715 2425.965
3 | 357.7333 888.5353 0.40 0.689 -1417.32 2132.787
5 | -158.5 1140.558 -0.14 0.890 -2437.026 2120.026
|
_cons | 6071.5 702.4489 8.64 0.000 4668.197 7474.803
------------------------------------------------------------------------------
这是有效的，因为传递给var asyncFuncCall = () => Promise.resolve();
function syncFunction() {
throw new Error("Fail");
}
asyncFuncCall()
.then(syncFunction)
.catch(e => console.log("Caught: " + e.message));的函数抛出的异常被转换为对它应该返回的promise的拒绝。
此外，传递给.then的函数返回的任何值都将转换为使用该值解析的promise。调用函数的promise代码负责处理这个问题。
这使您可以毫无问题地混合同步和异步代码：
.then