Javascript 从承诺返回 then()
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Return from a promise then()
提问by Priscy
I have got a javascript code like this:
我有一个这样的javascript代码：
function justTesting() {
promise.then(function(output) {
return output + 1;
});
}
var test = justTesting();
I have got always an undefined value for the var test. I think that it is because the promises are not resolved yet..there is a way to return a value from a promise?
我的 var 测试总是有一个未定义的值。我认为这是因为承诺还没有解决……有没有办法从承诺中返回一个值？
回答by c0ming
When you return something from a then()callback, it's a bit magic. If you return a value, the next then()is called with that value. However, if you return something promise-like, the next then()waits on it, and is only called when that promise settles (succeeds/fails).
当你从then()回调中返回一些东西时，这有点神奇。如果您返回一个值，then()则使用该值调用next 。但是，如果您返回类似 promise 的内容，则 nextthen()会等待它，并且仅在该 promise 解决（成功/失败）时调用。
回答by jfriend00
To use a promise, you have to either call a function that creates a promise or you have to create one yourself. You don't really describe what problem you're really trying to solve, but here's how you would create a promise yourself:
要使用承诺，您必须调用一个创建承诺的函数，或者您必须自己创建一个。你并没有真正描述你真正想要解决的问题，但这里是你自己创建承诺的方法：
function justTesting(input) {
return new Promise(function(resolve, reject) {
// some async operation here
setTimeout(function() {
// resolve the promise with some value
resolve(input + 10);
}, 500);
});
}
justTesting(29).then(function(val) {
// you access the value from the promise here
log(val);
});
// display output in snippet
function log(x) {
document.write(x);
}Or, if you already have a function that returns a promise, you can use that function and return its promise:
或者，如果您已经有一个返回承诺的函数，您可以使用该函数并返回其承诺：
// function that returns a promise
function delay(t) {
return new Promise(function(resolve) {
setTimeout(function() {
resolve();
}, t);
});
}
function justTesting(input) {
return delay(100).then(function() {
return input + 10;
});
}
justTesting(29).then(function(val) {
// you access the value from the promise here
log(val);
});
// display output in snippet
function log(x) {
document.write(x);
}回答by Vidur Singla
What I have done here is that I have returned a promise from the justTesting function. You can then get the result when the function is resolved.
我在这里所做的是我从 justTesting 函数返回了一个承诺。然后，您可以在解析函数时获得结果。
// new answer
function justTesting() {
return new Promise((resolve, reject) => {
if (true) {
return resolve("testing");
} else {
return reject("promise failed");
}
});
}
justTesting()
.then(res => {
let test = res;
// do something with the output :)
})
.catch(err => {
console.log(err);
});
Hope this helps!
希望这可以帮助！
// old answer
function justTesting() {
return promise.then(function(output) {
return output + 1;
});
}
justTesting().then((res) => {
var test = res;
// do something with the output :)
}
回答by beemaster
I prefer to use "await" command and async functions to get rid of confusions of promises,
我更喜欢使用“await”命令和异步函数来摆脱承诺的混乱，
In this case I would write an asynchronous function first, this will be used instead of the anonymous function called under "promise.then" part of this question :
在这种情况下，我将首先编写一个异步函数，这将用于代替在此问题的“promise.then”部分下调用的匿名函数：
async function SubFunction(output){
// Call to database , returns a promise, like an Ajax call etc :
const response = await axios.get( GetApiHost() + '/api/some_endpoint')
// Return :
return response;
}
and then I would call this function from main function :
然后我会从主函数调用这个函数：
async function justTesting() {
const lv_result = await SubFunction(output);
return lv_result + 1;
}
Noting that I returned both main function and sub function to async functions here.
请注意，我在这里将主函数和子函数都返回给了异步函数。
