未定义变量时-等待

Question

我有一个第一次从另一个地方自动触发的click事件。我的问题是它运行得太快了，因为所需的变量仍然由Flash和web服务定义。所以现在我有：

(function ($) {
    $(window).load(function(){
        setTimeout(function(){
            $('a.play').trigger("click");
        }, 5000);
    });
})(jQuery);

问题是，对于一个网速很慢的人来说，5秒可能太快了，反之亦然，对于一个网速很快的人来说，这太慢了。

那么在定义someVariable之前，我应该如何处理延迟或超时呢？

Answer 1

async, await实现，对@Toprak's answer的改进

(async() => {
    console.log("waiting for variable");
    while(!window.hasOwnProperty("myVar")) // define the condition as you like
        await new Promise(resolve => setTimeout(resolve, 1000));
    console.log("variable is defined");
})();
console.log("above code doesn't block main function stack");

在重新考虑了行动的问题之后。实际上有一种更好的方法来实现它的意图：“变量集回调”。虽然下面的代码仅在所需变量由对象(或窗口)封装而不是由let或var声明时才有效(我留下了第一个答案，因为我只是在对现有答案进行改进，而没有实际阅读原始问题)：

let obj = encapsulatedObject || window;
Object.defineProperty(obj, "myVar", {
    configurable: true,
    set(v){
        Object.defineProperty(obj, "myVar", {
            configurable: true, enumerable: true, writable: true, value: v });
        console.log("window.myVar is defined");
    }
});

请参阅Object.defineProperty或使用es6 proxy (这可能有些夸大其词)

如果您正在寻找更多信息：

​

/**
 * combining the two as suggested by @Emmanuel Mahuni,
 * and showing an alternative to handle defineProperty setter and getter
 */


let obj = {} || window;
(async() => {
  let _foo = await new Promise(res => {
    Object.defineProperty(obj, "foo", { set: res });
  });
  console.log("obj.foo is defined with value:", _foo);
})();
/*
IMPORTANT: note that obj.foo is still undefined
the reason is out of scope of this question/answer
take a research of Object.defineProperty to see more
*/

// TEST CODE

console.log("test start");
setTimeout(async () => {
  console.log("about to assign obj.foo");
  obj.foo = "Hello World!";
  // try uncomment the following line and compare the output
  // await new Promise(res => setTimeout(res));
  console.log("finished assigning obj.foo");
  console.log("value of obj.foo:", obj.foo); // undefined
  // console: obj.foo is defined with value: Hello World!
}, 2000);

​

Answer 2

下面的代码将继续查找someVariable，直到找到为止。它每0.25秒检查一次。

function waitForElement(){
    if(typeof someVariable !== "undefined"){
        //variable exists, do what you want
    }
    else{
        setTimeout(waitForElement, 250);
    }
}

Answer 3

我更喜欢这样的代码：

function checkVariable() {

   if (variableLoaded == true) {
       // Here is your next action
   }
 }

 setTimeout(checkVariable, 1000);