Make function wait until element exists

Question

I'm trying to add a canvas over another canvas – how can I make this function wait to start until the first canvas is created?

function PaintObject(brush) {

    this.started = false;

    // get handle of the main canvas, as a DOM object, not as a jQuery Object. Context is unfortunately not yet
    // available in jquery canvas wrapper object.
    var mainCanvas = $("#" + brush).get(0);

    // Check if everything is ok
    if (!mainCanvas) {alert("canvas undefined, does not seem to be supported by your browser");}
    if (!mainCanvas.getContext) {alert('Error: canvas.getContext() undefined !');}

    // Get the context for drawing in the canvas
    var mainContext = mainCanvas.getContext('2d');
    if (!mainContext) {alert("could not get the context for the main canvas");}

    this.getMainCanvas = function () {
        return mainCanvas;
    }
    this.getMainContext = function () {
        return mainContext;
    }

    // Prepare a second canvas on top of the previous one, kind of second "layer" that we will use
    // in order to draw elastic objects like a line, a rectangle or an ellipse we adjust using the mouse
    // and that follows mouse movements
    var frontCanvas = document.createElement('canvas');
    frontCanvas.id = 'canvasFront';
    // Add the temporary canvas as a second child of the mainCanvas parent.
    mainCanvas.parentNode.appendChild(frontCanvas);

    if (!frontCanvas) {
        alert("frontCanvas null");
    }
    if (!frontCanvas.getContext) {
        alert('Error: no frontCanvas.getContext!');
    }
    var frontContext = frontCanvas.getContext('2d');
    if (!frontContext) {
        alert("no TempContext null");
    }

    this.getFrontCanvas = function () {
        return frontCanvas;
    }
    this.getFrontContext = function () {
        return frontContext;
    }

Answer 1

If you have access to the code that creates the canvas - simply call the function right there after the canvas is created.

If you have no access to that code (eg. If it is a 3rd party code such as google maps) then what you could do is test for the existence in an interval:

var checkExist = setInterval(function() {
   if ($('#the-canvas').length) {
      console.log("Exists!");
      clearInterval(checkExist);
   }
}, 100); // check every 100ms

But note - many times 3rd party code has an option to activate your code (by callback or event triggering) when it finishes to load. That may be where you can put your function. The interval solution is really a bad solution and should be used only if nothing else works.

Answer 2

Depending on which browser you need to support, there's the option of MutationObserver.

EDIT: All major browsers support MutationObserver now.

Something along the lines of this should do the trick:

// callback executed when canvas was found
function handleCanvas(canvas) { ... }

// set up the mutation observer
var observer = new MutationObserver(function (mutations, me) {
  // `mutations` is an array of mutations that occurred
  // `me` is the MutationObserver instance
  var canvas = document.getElementById('my-canvas');
  if (canvas) {
    handleCanvas(canvas);
    me.disconnect(); // stop observing
    return;
  }
});

// start observing
observer.observe(document, {
  childList: true,
  subtree: true
});

N.B. I haven't tested this code myself, but that's the general idea.

You can easily extend this to only search the part of the DOM that changed. For that, use the mutations argument, it's an array of MutationRecord objects.

Answer 3

This will only work with modern browsers but I find it easier to just use a then so please test first but:

ES5

function rafAsync() {
    return new Promise(resolve => {
        requestAnimationFrame(resolve); //faster than set time out
    });
}

function checkElement(selector) {
    if (document.querySelector(selector) === null) {
        return rafAsync().then(() => checkElement(selector));
    } else {
        return Promise.resolve(true);
    }
}

ES6

async function checkElement(selector) {
    const querySelector = null;
    while (querySelector === null) {
        await rafAsync();
        querySelector = document.querySelector(selector);
    }
    return querySelector;
}  

Usage

checkElement('body') //use whichever selector you want
.then((element) => {
     console.info(element);
     //Do whatever you want now the element is there
});

Answer 4

A more modern approach to waiting for elements:

while(!document.querySelector(".my-selector")) {
  await new Promise(r => setTimeout(r, 500));
}
// now the element is loaded

Note that this code would need to be wrapped in an async function.

Answer 5

Here's a minor improvement over Jamie Hutber's answer

const checkElement = async selector => {
  while ( document.querySelector(selector) === null) {
    await new Promise( resolve =>  requestAnimationFrame(resolve) )
  }
  return document.querySelector(selector); 
};

To use:

checkElement('.myElement').then((selector) => {
  console.log(selector);
});

Answer 6

If you want a generic solution using MutationObserver you can use this function

// MIT Licensed
// Author: jwilson8767

/**
 * Waits for an element satisfying selector to exist, then resolves promise with the element.
 * Useful for resolving race conditions.
 *
 * @param selector
 * @returns {Promise}
 */
export function elementReady(selector) {
  return new Promise((resolve, reject) => {
    const el = document.querySelector(selector);
    if (el) {resolve(el);}
    new MutationObserver((mutationRecords, observer) => {
      // Query for elements matching the specified selector
      Array.from(document.querySelectorAll(selector)).forEach((element) => {
        resolve(element);
        //Once we have resolved we don't need the observer anymore.
        observer.disconnect();
      });
    })
      .observe(document.documentElement, {
        childList: true,
        subtree: true
      });
  });
}

Source: https://gist.github.com/jwilson8767/db379026efcbd932f64382db4b02853e
Example how to use it

elementReady('#someWidget').then((someWidget)=>{someWidget.remove();});

Note: MutationObserver has a great browser support; https://caniuse.com/#feat=mutationobserver

Et voilà ! :)

Answer 7

Is better to relay in requestAnimationFrame than in a setTimeout. this is my solution in es6 modules and using Promises.

es6, modules and promises:

// onElementReady.js
const onElementReady = $element => (
  new Promise((resolve) => {
    const waitForElement = () => {
      if ($element) {
        resolve($element);
      } else {
        window.requestAnimationFrame(waitForElement);
      }
    };
    waitForElement();
  })
);

export default onElementReady;

// in your app
import onElementReady from './onElementReady';

const $someElement = document.querySelector('.some-className');
onElementReady($someElement)
  .then(() => {
    // your element is ready
  }

plain js and promises:

var onElementReady = function($element) {
  return new Promise((resolve) => {
    var waitForElement = function() {
      if ($element) {
        resolve($element);
      } else {
        window.requestAnimationFrame(waitForElement);
      }
    };
    waitForElement();
  })
};

var $someElement = document.querySelector('.some-className');
onElementReady($someElement)
  .then(() => {
    // your element is ready
  });

Answer 8

Here is a solution using observables.

waitForElementToAppear(elementId) {                                          

    return Observable.create(function(observer) {                            
            var el_ref;                                                      
            var f = () => {                                                  
                el_ref = document.getElementById(elementId);                 
                if (el_ref) {                                                
                    observer.next(el_ref);                                   
                    observer.complete();                                     
                    return;                                                  
                }                                                            
                window.requestAnimationFrame(f);                             
            };                                                               
            f();                                                             
        });                                                                  
}                                                                            

Now you can write

waitForElementToAppear(elementId).subscribe(el_ref => doSomethingWith(el_ref);

Answer 9

You can check if the dom already exists by setting a timeout until it is already rendered in the dom.

var panelMainWrapper = document.getElementById('panelMainWrapper');
setTimeout(function waitPanelMainWrapper() {
    if (document.body.contains(panelMainWrapper)) {
        $("#panelMainWrapper").html(data).fadeIn("fast");
    } else {
        setTimeout(waitPanelMainWrapper, 10);
    }
}, 10);

Answer 10

Another variation of Iftah

var counter = 10;
var checkExist = setInterval(function() {
  console.log(counter);
  counter--
  if ($('#the-canvas').length || counter === 0) {
    console.log("by bye!");
    clearInterval(checkExist);
  }
}, 200);

Just in case the element is never shown, so we don't check infinitely.

Answer 11

A pure promise based JavaScript approach, you can tell for many milliseconds to wait.

    const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
        return new Promise((resolve) => {
            var waited = 0;
            var el = null;
            var wi = setInterval(function() {
                el = document.querySelector(query);
                if (waited >= ms || el) {
                    clearInterval(wi);
                    if(el) {
                        resolve(el);
                    } else {
                        resolve(null);
                    }
                }
                waited += 10;
            }, 10);  
        });
    }

To use the function, simply use the following code in an asynchronous function.

var element = await waitElementFor('#elementID');

Snippet:

const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
    return new Promise((resolve) => {
        var waited = 0;
        var el = null;
        var wi = setInterval(function() {
            el = document.querySelector(query);
            if (waited >= ms || el) {
                clearInterval(wi);
                if(el) {
                    resolve(el);
                } else {
                    resolve(null);
                }
            }
            waited += 10;
        }, 10);  
    });
}

async function snippetTestAyncFunction(){
    var element = await waitElementFor('#elementID');
    console.log(element);
}

snippetTestAyncFunction();

Answer 12

Maybe I'm a little bit late :), but here is a nice and brief solution by chrisjhoughton, which allows to perform a callback function when the wait is over.

https://gist.github.com/chrisjhoughton/7890303

var waitForEl = function(selector, callback) {
  if (jQuery(selector).length) {
    callback();
  } else {
    setTimeout(function() {
      waitForEl(selector, callback);
    }, 100);
  }
};

waitForEl(selector, function() {
  // work the magic
});

If you need to pass parameters to a callback function, you can use it this way:

waitForEl("#" + elDomId, () => callbackFunction(param1, param2));

But be careful! This solution by default can fall into a trap of an infinite loop.

Several improvements of the topicstarter's suggestion are also provided in The GitHub thread.

Enjoy!

Answer 13

This is for those of you who are running code in the Chrome console and not just hard-coded into the html.

user993683 above offered code that will work in your console code. His/her code is as follows:

while(!document.querySelector(".my-selector")) {
  await new Promise(r => setTimeout(r, 500));
}
// now the element is loaded

He/she added that it "needs to be inside an async function." And if you are using code in Chrome's console then in fact you DON'T need to wrap it in a function. It will work just as written. You only need to place it in your code at the place right before you try to access the element to make sure it exists.

The only caveat is that it won't work on elements that are only sometimes present under other circumstances. Otherwise it will loop indefinitely if the element never downloads and you'll have to close the browser to stop the wait. Only use it for elements which you are certain will be present.

My company's form page has a dozen or more fields to fill out for each case number. And I have hundreds of case numbers in the script array every day. The elements do not all load simultaneously when changing the iFrame SRC and "onload" does not work in Chrome console scripts. So this method is a god-send to me and it saves me at least 45 minutes every day over the old generic async wait 10 seconds here or 30 seconds there due to fluctuating load times.

The only change I made is "getElementById" instead of the general "querySelector" because all of the elements I need have ID's.

while(!document.getElementById("myFrame").contentWindow.document.getElementById('someDocID')) {
      await new Promise(r => setTimeout(r, 500));
    }
// After completing the wait above it is now safe to access the element
document.getElementById("myFrame").contentWindow.document.getElementById('someDocID'
).innerText = "Smith, John R";
// and now click the submit button then change the SRC to a fresh form, and use
//*emphasized text* the code again to wait for it to fully load

I apologize to the monitors, but I added this as an answer because after several months of research on console scripts and waiting for elements to load, user993683's remark about a function finally made me realize that console scripts do not require a function for this code. My goal here is only to save other consoler script users the same learning curve that I went through.

Answer 14

Just use setTimeOut with recursion:

waitUntilElementIsPresent(callback: () => void): void {
    if (!this.methodToCheckIfElementIsPresent()) {
        setTimeout(() => this.waitUntilElementIsPresent(callback), 500);
        return;
    }
    callback();
}

Usage:

this.waitUntilElementIsPresent(() => console.log('Element is present!'));

You can limit amount of attempts, so an error will be thrown when the element is not present after the limit:

waitUntilElementIsPresent(callback: () => void, attempt: number = 0): void {
    const maxAttempts = 10;
    if (!this.methodToCheckIfElementIsPresent()) {
        attempt++;
        setTimeout(() => this.waitUntilElementIsPresent(callback, attempt), 500);
        return;
    } else if (attempt >= maxAttempts) {
        return;
    }
    callback();
}