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Waiting for an element to be created

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[前端(javascript) 所属分类 前端(javascript) | 发布者 店小二03 | 时间 2016 | 作者 红领巾 ] 0人收藏点击收藏

In my trials and tribulations to detect when a field has been autofilled, I need to create a shim for monitorEvents so that I can see the event lifecycle of that element and ultimately try to debug it.

One thing that I found is that monitorEvents requires an element but for what I am doing I know that there will be an element with an id at somepoint but I don’t know when it will be created.

I quickly knocked out a small function called waitForElement that uses the MutationObserver to look for when an element with a given id is added to the DOM. When that element has been detected it will resolve the promise and return the element.

The code is as follows:

function waitForElement(id) { return new Promise(function(resolve, reject) { var elementId = id.substr(1); var element = document.getElementById(elementId); if(element) { resolve(element); return; } var observer = new MutationObserver(function(mutations) { mutations.forEach(function(mutation) { var nodes = Array.from(mutation.addedNodes); nodes.forEach(function(node) { if(node.id == elementId) { observer.disconnect(); resolve(node); return; } }); }); }); observer.observe(document.documentElement, { childList: true, subtree: true }); }); }

Here is the gist if that is your bag .

It is pretty simple to use this simple API.

waitForElement("#test").then(function(element) { console.log("Element Added", element); });

Now combining in the monitorEvents function from my previous post, I can now set a breakpoint early in the lifecycle of a page (because scripts in the head block) and set up a waitForElement call that can now start logging all the events that are firing on that element.

waitForElement("#test").then(function(element) { monitorEvents(element); });

Technically I still haven’t solved the issue of “how can you tell when Firefox has autocompleted fields” but I have the tools at my disposal.

Pretty chuffed.

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