虽然变量没有定义-等待

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Instead of using the windows load event use the ready event on the document.

$(document).ready(function(){[...]});

This should fire when everything in the DOM is ready to go, including media content fully loaded.

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The following will keep looking for someVariable until it is found. It checks every 0.25 seconds.

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

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You could have Flash call the function when it's done. I'm not sure what you mean by web services. I assume you have JavaScript code calling web services via Ajax, in which case you would know when they terminate. In the worst case, you could do a looping setTimeout that would check every 100 ms or so.

And the check for whether or not a variable is defined can be just if (myVariable) or safer: if(typeof myVariable == "undefined")

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I would prefer this code:

function checkVariable() {


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


setTimeout(checkVariable, 1000);

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You can use this:

var refreshIntervalId = null;
refreshIntervalId = setInterval(checkIfVariableIsSet, 1000);


var checkIfVariableIsSet = function()
{
if(typeof someVariable !== 'undefined'){
$('a.play').trigger("click");
clearInterval(refreshIntervalId);
}
};

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Shorter way:

   var queue = function (args){
typeof variableToCheck !== "undefined"? doSomething(args) : setTimeout(function () {queue(args)}, 2000);
};

You can also pass arguments

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Here's an example where all the logic for waiting until the variable is set gets deferred to a function which then invokes a callback that does everything else the program needs to do - if you need to load variables before doing anything else, this feels like a neat-ish way to do it, so you're separating the variable loading from everything else, while still ensuring 'everything else' is essentially a callback.

var loadUser = function(everythingElse){
var interval = setInterval(function(){
if(typeof CurrentUser.name !== 'undefined'){
$scope.username = CurrentUser.name;
clearInterval(interval);
everythingElse();
}
},1);
};


loadUser(function(){


//everything else


});

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With Ecma Script 2017 You can use async-await and while together to do that And while will not crash or lock the program even variable never be true

//First define some delay function which is called from async function
function __delay__(timer) {
return new Promise(resolve => {
timer = timer || 2000;
setTimeout(function () {
resolve();
}, timer);
});
};


//Then Declare Some Variable Global or In Scope
//Depends on you
let Variable = false;


//And define what ever you want with async fuction
async function some() {
while (!Variable)
await __delay__(1000);


//...code here because when Variable = true this function will
};
////////////////////////////////////////////////////////////
//In Your Case
//1.Define Global Variable For Check Statement
//2.Convert function to async like below


var isContinue = false;
setTimeout(async function () {
//STOPT THE FUNCTION UNTIL CONDITION IS CORRECT
while (!isContinue)
await __delay__(1000);


//WHEN CONDITION IS CORRECT THEN TRIGGER WILL CLICKED
$('a.play').trigger("click");
}, 1);
/////////////////////////////////////////////////////////////

Also you don't have to use setTimeout in this case just make ready function asynchronous...

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I have upvoted @dnuttle's answer, but ended up using the following strategy:

// On doc ready for modern browsers
document.addEventListener('DOMContentLoaded', (e) => {
// Scope all logic related to what you want to achieve by using a function
const waitForMyFunction = () => {
// Use a timeout id to identify your process and purge it when it's no longer needed
let timeoutID;
// Check if your function is defined, in this case by checking its type
if (typeof myFunction === 'function') {
// We no longer need to wait, purge the timeout id
window.clearTimeout(timeoutID);
// 'myFunction' is defined, invoke it with parameters, if any
myFunction('param1', 'param2');
} else {
// 'myFunction' is undefined, try again in 0.25 secs
timeoutID = window.setTimeout(waitForMyFunction, 250);
}
};
// Initialize
waitForMyFunction();
});

It is tested and working! ;)

Gist: https://gist.github.com/dreamyguy/f319f0b2bffb1f812cf8b7cae4abb47c

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Object.defineProperty(window, 'propertyName', {
set: value => {
this._value = value;
// someAction();
},
get: () => this._value
});

or even if you just want this property to be passed as an argument to a function and don't need it to be defined on a global object:

Object.defineProperty(window, 'propertyName', { set: value => someAction(value) })

However, since in your example you seem to want to perform an action upon creation of a node, I would suggest you take a look at MutationObservers.

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最佳答案

async, await implementation, improvement over @Toprak's answer

(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");

After revisiting the OP's question. There is actually a better way to implement what was intended: "variable set callback". Although the below code only works if the desired variable is encapsulated by an object (or window) instead of declared by let or var (I left the first answer because I was just doing improvement over existing answers without actually reading the original question):

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");
}
});

see Object.defineProperty or use es6 proxy (which is probably overkill)

If you are looking for more:

/**
* 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);

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I prefer something simple like this:

function waitFor(variable, callback) {
var interval = setInterval(function() {
if (window[variable]) {
clearInterval(interval);
callback();
}
}, 200);
}

And then to use it with your example variable of someVariable:

waitFor('someVariable', function() {
// do something here now that someVariable is defined
});

Note that there are various tweaks you can do. In the above setInterval call, I've passed 200 as how often the interval function should run. There is also an inherent delay of that amount of time (~200ms) before the variable is checked for -- in some cases, it's nice to check for it right away so there is no delay.

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Very late to the party but I want to supply a more modern solution to any future developers looking at this question. It's based off of Toprak's answer but simplified to make it clearer as to what is happening.

<div>Result: <span id="result"></span></div>


<script>
var output = null;
    

// Define an asynchronous function which will not block where it is called.
async function test(){
// Create a promise with the await operator which instructs the async function to wait for the promise to complete.
await new Promise(function(resolve, reject){
// Execute the code that needs to be completed.
// In this case it is a timeout that takes 2 seconds before returning a result.
setTimeout(function(){
// Just call resolve() with the result wherever the code completes.
resolve("success output");
}, 2000);
            

// Just for reference, an 'error' has been included.
// It has a chance to occur before resolve() is called in this case, but normally it would only be used when your code fails.
setTimeout(function(){
// Use reject() if your code isn't successful.
reject("error output");
}, Math.random() * 4000);
})
.then(function(result){
// The result variable comes from the first argument of resolve().
output = result;
})
.catch(function(error){
// The error variable comes from the first argument of reject().
// Catch will also catch any unexpected errors that occur during execution.
// In this case, the output variable will be set to either of those results.
if (error) output = error;
});
        

// Set the content of the result span to output after the promise returns.
document.querySelector("#result").innerHTML = output;
}
    

// Execute the test async function.
test();


// Executes immediately after test is called.
document.querySelector("#result").innerHTML = "nothing yet";
</script>

Here's the code without comments for easy visual understanding.

var output = null;


async function test(){
await new Promise(function(resolve, reject){
setTimeout(function(){
resolve("success output");
}, 2000);


setTimeout(function(){
reject("error output");
}, Math.random() * 4000);
})
.then(function(result){
output = result;
})
.catch(function(error){
if (error) output = error;
});


document.querySelector("#result").innerHTML = output;
}


test();


document.querySelector("#result").innerHTML = "nothing yet";

On a final note, according to MDN, Promises are supported on all modern browsers with Internet Explorer being the only exception. This compatibility information is also supported by caniuse. However with Bootstrap 5 dropping support for Internet Explorer, and the new Edge based on webkit, it is unlikely to be an issue for most developers.

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while (typeof myVar == void(0)) {
if ( typeof myVar != void(0)) {
console.log(myVar);
}
}

This makes use of the typeof operator which only returns undefined if variable is not declared yet. Should be applicable in every type of javascript.

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I have an adaptation of the answer by @dnuttle that I would suggest using.

The advantage of using a try-catch block is that if any part of the code you are trying to execute fails, the whole block fails. I find this useful because it gives you a kind of transaction; everything or nothing gets done.

You should never write code that could end up in an endless loop due to external factors. This is exactly what would happen if you were waiting for a response from an ajax request and the server doesn't respond. I think it's good practice to have a timeout for any questionable loops.

let time = 0; // Used to keep track of how long the loop runs
function waitForElement() {
try {
// I'm testing for an element, but this condition can be
// any reasonable condition
if (document.getElementById('test') === null) {
throw 'error';
}


// This is where you can do something with your variable
// document.getElementById('test').addEventListener....
// or call a function that uses your value


} catch (error) {
// Loop stops executing if not successful within about 5 seconds
if (time > 5000) {
// I return false on failure so I can easily check for failure
return false;
} else {
// Increment the time and call the function again
time += 250;
setTimeout(waitForElement, 250);
}
}
}


// Call the function after the definition, ensures that time is set
waitForElement();