JavaScript检查资源是否可以通过提取访问 [英] JavaScript checking if resource is reachable with fetch
问题描述
我基本上只是试图验证资源是否可以从正在执行的客户端访问。我不能使用 XHR ，因为目标资源不允许。
我很新JS和我目前正在使用（可执行文件）：
var done = false;
var i = 1;
var t =https://i.stack.imgur.com/Ya15i.jpg; $！
$ b while（！done&& i< 4）
{
console.log（try+ i）;
完成= chk（t）;
sleep（1000）;
i = i + 1;
if（done）
{
console.log（Reachable！）;
休息;
}
else
{
console.log（Unreachable。）;
函数chk（target）
{
console.log（checking+ target）
fetch（target， {mode：'no-cors'}）。then（r => {
return true;
}）
.catch（e => {
return false;
}）;
}
//忙碌假睡眠
函数sleep（s）
{
var now = new Date（）。getTime（）;
while（new Date（）。getTime（）< now + s）{/ * busy sleep * /}
}
我期待这段代码检查资源，打印结果，然后等待一秒。重复此操作直到3次尝试失败或者其中一次成功。
取而代之的是执行块，然后打印所有控制台。我知道 fetch / code>操作是异步的，但我想如果我以前声明 done 并实现它应该工作的睡眠。在最糟糕的情况下，while循环将使用先前声明的 done 。
如何实现所描述的行为？任何建议都是可以接受的。
你真的想要的是一个递归函数，在延迟 y 秒等检查url n 次数后返回一个promise。 。类似于这个
函数chk（target，times，延迟）{
return new Promise（（res，rej）=> {// return a promise
（function rec（i）{//递归IIFE
fetch目标，{mode：'no-cors'}）。then（（r）=> {//获取资源
res（r）; //解析promise如果成功
}）。catch （err => {
if（times === 0）//如果尝试次数达到
return rej（err）; //不要再次尝试
SE tTimeout（（）=> rec（ - 次），延迟）//否则，请等待并尝试
}）; //再次尝试
}）（times）;
}）;
}
像这样使用
var t =https://i.stack.imgur.com/Ya15i.jpg;
chk（t，3，1000）.then（image => {
console.log（'success'）
}）。catch（err => {
console.log（'error'）
}）;
请注意，这不会在404或500上失败，任何响应都是成功的请求。 p>
I'm basically just trying to verify if a resource is reachable from the executing client. I can not use XHR, because the target resource doesn't allow that.
I'm pretty new to JS and am currently working with this ( executable here ):
var done = false;
var i = 1;
var t = "https://i.stack.imgur.com/Ya15i.jpg";
while(!done && i < 4)
{
console.log("try "+i);
done = chk(t);
sleep(1000);
i = i+1;
if (done)
{
console.log("Reachable!");
break;
}
else
{
console.log("Unreachable.");
}
}
function chk(target)
{
console.log("checking "+target)
fetch(target, {mode: 'no-cors'}).then(r=>{
return true;
})
.catch(e=>{
return false;
});
}
// busy fake sleep
function sleep(s)
{
var now = new Date().getTime();
while(new Date().getTime() < now + s){ /* busy sleep */ }
}
I was expecting this code to check for the resource, print the result, then wait for a sec. Repeat this until 3 tries were unsuccessful or one of them was successful.
Instead the execution blocks for a while, then prints all of the console.logs at once and the resource is never reachable (which it is).
I do know that the fetch operation is asynchronous, but I figured if I previously declare done and implement a sleep it should work. In the worst case, the while loop would use the previously declared done.
How do I achieve the described behavior? Any advice is welcome.
Your sleep function is blocking, what you really want is a recursive function that returns a promise after checking the url n times with a delay of y seconds etc.
Something like this
function chk(target, times, delay) {
return new Promise((res, rej) => { // return a promise
(function rec(i) { // recursive IIFE
fetch(target, {mode: 'no-cors'}).then((r) => { // fetch the resourse
res(r); // resolve promise if success
}).catch( err => {
if (times === 0) // if number of tries reached
return rej(err); // don't try again
setTimeout(() => rec(--times), delay ) // otherwise, wait and try
}); // again until no more tries
})(times);
});
}
To be used like this
var t = "https://i.stack.imgur.com/Ya15i.jpg";
chk(t, 3, 1000).then( image => {
console.log('success')
}).catch( err => {
console.log('error')
});
And note that this does not fail on 404 or 500, any response is a successful request.
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