I have a click event that is triggered from another place automatically for the first time. My problem is that it runs too soon, since the required variables are still being defined by Flash and web services. So right now I have:
(function ($) {
$(window).load(function(){
setTimeout(function(){
$('a.play').trigger("click");
}, 5000);
});
})(jQuery);
The problem is that 5 seconds for a person with a slow internet connection could be too fast and vice versa, for a person with a fast internet connection, it's too slow.
So how should I do the delay or timeout until someVariable is defined?
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);
}
}
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);
I would prefer this code:
function checkVariable() {
if (variableLoaded == true) {
// Here is your next action
}
}
setTimeout(checkVariable, 1000);
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.
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...
You can use this:
var refreshIntervalId = null;
refreshIntervalId = setInterval(checkIfVariableIsSet, 1000);
var checkIfVariableIsSet = function()
{
if(typeof someVariable !== 'undefined'){
$('a.play').trigger("click");
clearInterval(refreshIntervalId);
}
};
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
});
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.
Shorter way:
var queue = function (args){
typeof variableToCheck !== "undefined"? doSomething(args) : setTimeout(function () {queue(args)}, 2000);
};
You can also pass arguments
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
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.
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();
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")
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.
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.
I have an interceptor function that I can put some javascript code into (for swagger ui). I only have one place to put in javascript code and it will be re-run frequently. I need to add a mutating observer in there, and have it only be setup one time.
There are many examples of how to do a Javascript method that will allow your code to be only run once. Here is a popular example: Function in JavaScript that can be called only once
But it assumes that you have somewhere to put the function where it will only be called once.
Is there a way to have my javascript code (not necessarily a function) only be run once? (So I don't end up setting up a bunch of mutating observers?)
To illustrate with an example, here is the code from the question I linked to, but copied in twice to show that it would be run many times:
var something = (function() {
var executed = false;
return function() {
if (!executed) {
executed = true;
console.log("hello");
}
};
})();
something(); // "do something" happens
something(); // nothing happens
var something = (function() {
var executed = false;
return function() {
if (!executed) {
executed = true;
console.log("hello");
}
};
})();
something(); // "do something" happens
something(); // nothing happens
The output of this code is:
hello
hello
Because the function is initalized twice, the call to console.log happens twice.
I need some way to have my code only happen once, with only one place to declare it.
What about storing executed value in document ... it is created once at execution ... it could be stored in window (i.e. global var) but I think it is better to be kept in document. I'm not familiar with swagger but docuument should always exits I think. Storing it in localStorage will keep it longer than you may need and you should reset it on each run.
var something = (function() {
return function() {
if (!document.executed) {
document.executed = true;
console.log("hello");
}
};
})();
something(); // "do something" happens
something(); // nothing happens
var something = (function() {
return function() {
if (!document.executed) {
document.executed = true;
console.log("hello");
}
};
})();
something(); // "do something" happens
something(); // nothing happens
For browser environment, you can use localStorage to set the value.
// make sure to initialize with true in your main entry point, otherwise the function would not run for the first time
localStorage.setItem('run-function', true);
const run = localStorage.getItem('run-function');
function test() {
if (run) {
console.log('ran function');
// ran once, set the value to false to prevent dupes
localStorage.setItem('run-function', false);
} else {
console.log('function already called once, exiting');
}
}
test();
if you're on node.js environment you can make use of the global variable
// make sure to initialize with true in your main entry point, otherwise the function would not run for the first time
global.run_function = true;
function test() {
if (global.run_function) {
console.log('ran function');
global.run_function = false;
} else {
console.log('function already called once, exiting');
}
}
test();
Is there a way to do this in JS
function namedFunction(elements,args) {
const domElements = document.querySelector(elements);
const initialValue = 0;
let incrementBy = 5;
return function() {
// Do something to domElements based on initialValue and incrementBy
// function needs to run the first time namedFunction is called
// and this is the only function that needs to run on subsequent calls to namedFunction
}.call(null)
// the .call does not work as intended here, but this is basically what I want to do.
}
I think I can do namedFunction()() with the code above in order to invoke both, but I'm wondering if there is another way.
The longer version of the function would look like this:
function namedFunction(elements,args) {
const domElements = document.querySelector(elements);
const initialValue = 0;
let incrementBy = 5;
function namedFunctionEventHandler() {
// Do something to domElements based on initialValue and incrementBy
// function needs to run the first time namedFunction is called
// and this is the only function that needs to run on subsequent calls to namedFunction
}
namedFunctionEventHandler();
return namedFunctionEventHandler;
}
The goal would be to pass a single function as an event handler, that the first time it runs it does initial calculations, caches dom elements and the more heavier stuff, then executes the logic that is abstracted in the returned function and on subsequent calls it uses the data from the closure.
Edit: the namedFunction does not need to accept any arguments, its just for demonstration purposes.
document.addEventListener('scroll', namedFunction)
is what I want to be able to do.
#CertainPerformance - Sorry, I misread your answer.
If you take a look at the end result I would like to achieve, your proposition wont actually work as intended, as if I pass an invoked function as an event handler, its gonna run before an event has actually happened.
You can make namedFunction into an IIFE that saves a reference to a function (initially undefined). On call, if that variable is undefined, carry out the expensive calculations and then assign to the variable; if the variable is defined, then simply call it.
const handler = (() => {
let cheapFn;
return () => {
if (cheapFn) {
cheapFn();
return;
}
// expensive calculations here
const domElements = document.querySelector(elements);
...
cheapFn = () => {
// assign to cheapFn
};
cheapFn();
};
})();
Demo:
const handler = (() => {
let cheapFn;
return () => {
if (cheapFn) {
cheapFn();
return;
}
// expensive calculations here
console.log('expensive');
cheapFn = () => {
console.log('cheap');
};
cheapFn();
};
})();
document.addEventListener('scroll', handler);
body {
height: 400px;
}
body
You can take advantage of the fact that functions in JavaScript are first-class objects, and store the function state (initialized/uninitialized) in a property of the function.
The data computed during initialization can be stored in the function properties as well, please take a look at the demo:
const namedFunction = function(elements,args) {
if (!namedFunction.isInitialized) {
console.log('Initialization: Computing initial value...');
namedFunction.initialValue = 10 * 10;
console.log(`Initial value: ${namedFunction.initialValue}`);
namedFunction.isInitialized = true;
}
return function() {
console.log('Running regular operation:');
console.log(`Current value: ${--namedFunction.initialValue}`);
}.call(null)
}
document.getElementById('demo').addEventListener('click', namedFunction);
<button id="demo">Run</button>
I have a problem with a singleton pattern that I have implemented in 'program.js':
var Program = (function() {
var _program; // Instance of program
// Constructor
function Program() {
if (typeof _program != "undefined") {
throw new Error("Program can only be instantiated once.");
}
this.run = false; // flag to allow for program exec
_program = this;
};
// Public methods
Program.prototype.init = function() {
// the run call is nested in 4 callbacks
callbackHell (
this.run = true;
);
};
Program.prototype.execute = function() {
if(this.run == true) {
// do stuff
}
};
Program.getProgram = function () {
if(typeof _program == "undefined")
{
return new this();
}
return _program;
};
// Return the constructor
return Program;
})();
In my 'main.js' I keep a reference to the loaded program and check for the run flag to allow execution. Looks like this:
var program = null;
function initDemo() {
program = Program.getProgram();
program.init();
runDemo();
};
function runDemo() {
if(program != null && program.run) {
program.execute();
}
requestAnimationFrame(runDemo);
};
If I execute this code on a Chrome browser, it will never reach the program.execute() call in main.js. The reference of program will keep the run flag to false, even it is changed in the init() function. I checked for all of this in the debugger. I should point out that the 'this.run = true' call is nested in 4 callbacks. After a while I figured I could just change the run flag of the global reference of program in main.js with the init() function. So instead of 'this.run = true', 'program.run = true'. This works and the loop will run execute(). However this is not the style I am used from OOP. What is actually happening here? It definitely has to do with the callbacks: when I put 'this.run = true' out of the callbacks at the end of init() the flag is changed correctly, however at the wrong time of the program execution.
Probably your callbacks in the callbackHell are doing something asynchronously and there is a delay before program.run will actually be set to true, the sequence is approximately this:
You call program.init()
Your callbacks start working
You call runDemo(), here program.run is false and it exists
Callbacks finish their work and program.run becomes true
The solution to that is to make your runDemo to be another callback, so your main code will look like this:
var program = null;
function initDemo() {
program = Program.getProgram();
program.init(runDemo);
};
function runDemo() {
if(program != null) {
program.execute();
}
requestAnimationFrame(runDemo);
};
Here you don't need the program.run flag at all, instead you just start your demo from inside the "callbackHell":
Program.prototype.init = function(functionToRun) {
// the run call is nested in 4 callbacks
callbackHell (
functionToRun(); // instead of setting program.run
// we call the specified function
);
};
I am looking for a good technique to get away from what I am tempted to do: to set a global variable.
The first time someone runs a function by clicking a button it triggers an initial function to turn a few things into draggables. Later, if they click the button a second time I want to determine if the init function has been initialized, and if so to not call it again. I could easily do this by setting a global variable from the init function and then checking that variable from the click function, but I'm wondering how to do this without setting a global variable. I would really like an example of a way to do this.
You could add a property to the function:
function init() {
init.called = true;
}
init();
if(init.called) {
//stuff
}
While #Levi's answer ought to work just fine, I would like to present another option. You would over write the init function to do nothing once it has been called.
var init = function () {
// do the initializing
init = function() {
return false;
}
};
The function when called the first time will do the init. It will then immediately overwrite itself to return false the next time its called. The second time the function is called, the function body will only contain return false.
For more reading: http://www.ericfeminella.com/blog/2011/11/19/function-overwriting-in-javascript/
Why don't you just check to see if your draggables have a class of draggable on them?
if ($('.mydiv').is('.draggable')) {
//do something
}
Function.prototype.fired = false;
function myfunc() {
myfunc.fired = true;
// your stuff
};
console.log(myfunc.fired) // false
myfunc();
console.log(myfunc.fired) // true
What you could do is unhook the init function from the prototype.
var Obj = function () {
this.init = function () {
document.write("init called<br/>");
this.init = null;
}
}
var o = new Obj();
if (o.init) document.write("exists!<br/>");
o.init();
if (o.init) document.write("exists!<br/>");
o.init();
The first if will be true and print exists! but since the function removes itself, the second if will fail. In my example, I call the second init unconditionally just to show that nothing will happen, but of course you could call it only if it exists:
if (o.init) o.init();
http://jsfiddle.net/coreyog/Wd3Q2/
The correct approach is to use the Javascript Proxy APIs to trap the function calls using apply handler.
const initFun = (args) => {
console.log('args', args);
}
const init = new Proxy(initFun, {
apply(target, thisArg, args){
target.calls = target.calls ? target.calls + 1 : 1;
return target.apply(thisArg, args);
}
});
init('hi');
console.log(init.calls); // 1
init('hello');
console.log(init.calls); // 2