How to get the return value of JavaScript asynchronous function
Study a small question today: How to get the return value of the JavaScriptasynchronous function?
1. Wrong attempts
When I didn’t enter the industry, my first attempt was:
<script>
function getSomething() {
var r = 0;
setTimeout(function() {
r = 2;
}, 10);
return r;
}
function compute() {
var x = getSomething();
alert(x * 2);
}
compute();
</script>
2. Callback function
What pops up is not 4, but 0. Later I found out that this is an asynchronous problem,
We need to use callback technology to do it:
<script>
function getSomething(cb) {
var r = 0;
setTimeout(function() {
r = 2;
cb(r);
}, 10);
}
function compute(x) {
alert(x * 2);
}
getSomething(compute);
</script>
3.promise
The callback function is really a good thing, and I have been writing code like this for a long time. Pass the function when encountering asynchronous! ! Later, I learned that there is a thing called promise, which specifically solves problems caused by callback functions. I also learned about promise:
<script>
function getSomething() {
var r = 0;
return new Promise(function(resolve) {
setTimeout(function() {
r = 2;
resolve(r);
}, 10);
});
}
function compute(x) {
alert(x * 2);
}
getSomething().then(compute);
</script>
promise still does not give up the callback, it just happens at the location where the callback occurs. changed.
4.generator
Later I learned about generator and knew that it has the ability to interrupt function execution, so I made a new attempt:
<script>
function getSomething() {
var r = 0;
setTimeout(function() {
r = 2;
it.next(r);
}, 10);
}
function *compute(it) {
var x = yield getSomething();
alert(x * 2);
}
var it = compute();
it.next();
</script>
The synchronous writing method can realize asynchronous logic, which feels much more advanced.
5.promise + generator
Later I heard that promise plus generator is the perfect way to asynchronously use anti-aircraft guns to kill mosquitoes (this example is not enough Tell me the benefits of using the two together):
<script>
function getSomething() {
var r = 0;
return new Promise(function(resolve) {
setTimeout(function() {
r = 2;
resolve(r);
}, 10);
});
}
function *compute() {
var x = yield getSomething();
alert(x * 2);
}
var it = compute();
it.next().value.then(function(value) {
it.next(value);
});
</script>
6.async
I think this is cool enough. , and later I heard that es7 gave the ultimate solution: async.
As a young man who loves to learn, I thought that I could not be left behind:
<script>
function getSomething() {
var r = 0;
return new Promise(function(resolve) {
setTimeout(function() {
r = 2;
resolve(r);
}, 10);
});
}
async function compute() {
var x = await getSomething();
alert(x * 2);
}
compute();
</script>
I finally breathed a sigh of relief here.
Postscript:
All the above examples can be run on the latest chrome.
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