下面是一个使用MutationObserver api的简单解决方案。

没有jQuery 没有计时器 没有第三方库 基于Promise，并与async/await一起工作

我在几个项目中使用过它。

function waitForElm(selector) {
    return new Promise(resolve => {
        if (document.querySelector(selector)) {
            return resolve(document.querySelector(selector));
        }

        const observer = new MutationObserver(mutations => {
            if (document.querySelector(selector)) {
                resolve(document.querySelector(selector));
                observer.disconnect();
            }
        });

        observer.observe(document.body, {
            childList: true,
            subtree: true
        });
    });
}

使用它:

waitForElm('.some-class').then((elm) => {
    console.log('Element is ready');
    console.log(elm.textContent);
});

或者使用async/await:

const elm = await waitForElm('.some-class');

下面是一个函数，充当MutationObserver的薄包装器。唯一的要求是浏览器支持MutationObserver;不依赖于JQuery。运行下面的代码片段以查看一个工作示例。

function waitForMutation(parentNode, isMatchFunc, handlerFunc, observeSubtree, disconnectAfterMatch) { var defaultIfUndefined = function(val, defaultVal) { return (typeof val === "undefined") ? defaultVal : val; }; observeSubtree = defaultIfUndefined(observeSubtree, false); disconnectAfterMatch = defaultIfUndefined(disconnectAfterMatch, false); var observer = new MutationObserver(function(mutations) { mutations.forEach(function(mutation) { if (mutation.addedNodes) { for (var i = 0; i < mutation.addedNodes.length; i++) { var node = mutation.addedNodes[i]; if (isMatchFunc(node)) { handlerFunc(node); if (disconnectAfterMatch) observer.disconnect(); }; } } }); }); observer.observe(parentNode, { childList: true, attributes: false, characterData: false, subtree: observeSubtree }); } // Example waitForMutation( // parentNode: Root node to observe. If the mutation you're looking for // might not occur directly below parentNode, pass 'true' to the // observeSubtree parameter. document.getElementById("outerContent"), // isMatchFunc: Function to identify a match. If it returns true, // handlerFunc will run. // MutationObserver only fires once per mutation, not once for every node // inside the mutation. If the element we're looking for is a child of // the newly-added element, we need to use something like // node.querySelector() to find it. function(node) { return node.querySelector(".foo") !== null; }, // handlerFunc: Handler. function(node) { var elem = document.createElement("div"); elem.appendChild(document.createTextNode("Added node (" + node.innerText + ")")); document.getElementById("log").appendChild(elem); }, // observeSubtree true, // disconnectAfterMatch: If this is true the hanlerFunc will only run on // the first time that isMatchFunc returns true. If it's false, the handler // will continue to fire on matches. false); // Set up UI. Using JQuery here for convenience. $outerContent = $("#outerContent"); $innerContent = $("#innerContent"); $("#addOuter").on("click", function() { var newNode = $("<div><span class='foo'>Outer</span></div>"); $outerContent.append(newNode); }); $("#addInner").on("click", function() { var newNode = $("<div><span class='foo'>Inner</span></div>"); $innerContent.append(newNode); }); .content { padding: 1em; border: solid 1px black; overflow-y: auto; } #innerContent { height: 100px; } #outerContent { height: 200px; } #log { font-family: Courier; font-size: 10pt; } <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <h2>Create some mutations</h2> <div id="main"> <button id="addOuter">Add outer node</button> <button id="addInner">Add inner node</button> <div class="content" id="outerContent"> <div class="content" id="innerContent"></div> </div> </div> <h2>Log</h2> <div id="log"></div>

对于一个使用jQuery的简单方法，我发现这工作得很好:

  // Wait for element to exist.
  function elementLoaded(el, cb) {
    if ($(el).length) {
      // Element is now loaded.
      cb($(el));
    } else {
      // Repeat every 500ms.
      setTimeout(function() {
        elementLoaded(el, cb)
      }, 500);
    }
  };

  elementLoaded('.element-selector', function(el) {
    // Element is ready to use.
    el.click(function() {
      alert("You just clicked a dynamically inserted element");
    });
  });

在这里，我们只需每500毫秒检查一次元素是否加载，当加载成功时，我们就可以使用它。

这对于向已动态添加到文档中的元素添加单击处理程序特别有用。

这里有一个用Javascript编写的promise返回解决方案(没有混乱的回调)。默认情况下，它每200ms检查一次。

function waitFor(selector) {
    return new Promise(function (res, rej) {
        waitForElementToDisplay(selector, 200);
        function waitForElementToDisplay(selector, time) {
            if (document.querySelector(selector) != null) {
                res(document.querySelector(selector));
            }
            else {
                setTimeout(function () {
                    waitForElementToDisplay(selector, time);
                }, time);
            }
        }
    });
}

你也可以使用getElementById代替querySelector

这个函数和 https://stackoverflow.com/a/61511955/10798137

  async function waitForElementById(id, timeout = null, location = document.body) {
    return new Promise((resolve) => {
        let element = document.getElementById(id);
        if (element) {
            return resolve(element);
        }

        const observer = new MutationObserver(async () => {
            let element = document.getElementById(id);
            if (element) {
                resolve(element);
                observer.disconnect();
            } else {
                if (timeout) {
                    async function timeOver() {
                        return new Promise((resolve) => {
                            setTimeout(() => {
                                observer.disconnect();
                                resolve(false);
                            }, timeout);
                        });
                    }
                    resolve(await timeOver());
                }
            }
        });

        observer.observe(location, {
            childList: true,
            subtree: true,
        });
    });

使用它

    waitForElement("tag_id", 500).then((elm) => {
      console.log(elm)
    })

Or

    var elm = async waitForElement("tag_id", 500)

这是一个纯Javascript函数，它允许你等待任何事情。设置更长的间隔，以占用更少的CPU资源。

/**
 * @brief Wait for something to be ready before triggering a timeout
 * @param {callback} isready Function which returns true when the thing we're waiting for has happened
 * @param {callback} success Function to call when the thing is ready
 * @param {callback} error Function to call if we time out before the event becomes ready
 * @param {int} count Number of times to retry the timeout (default 300 or 6s)
 * @param {int} interval Number of milliseconds to wait between attempts (default 20ms)
 */
function waitUntil(isready, success, error, count, interval){
    if (count === undefined) {
        count = 300;
    }
    if (interval === undefined) {
        interval = 20;
    }
    if (isready()) {
        success();
        return;
    }
    // The call back isn't ready. We need to wait for it
    setTimeout(function(){
        if (!count) {
            // We have run out of retries
            if (error !== undefined) {
                error();
            }
        } else {
            // Try again
            waitUntil(isready, success, error, count -1, interval);
        }
    }, interval);
}

要调用它，例如在jQuery中，使用如下代码:

waitUntil(function(){
    return $('#myelement').length > 0;
}, function(){
    alert("myelement now exists");
}, function(){
    alert("I'm bored. I give up.");
});