What jQuery returns if it cannot find the object

In front-end development, we often use the jQuery library to operate DOM elements, send Ajax requests and other operations. But when using jQuery, you sometimes encounter some problems, such as not being able to find an object or element. What does jQuery return if it cannot find the object?

First of all, you need to understand the return value type of jQuery selector. The jQuery selector returns a jQuery object, which is a collection of matched DOM elements. If the selector matches multiple elements, the jQuery object contains multiple elements. If no elements are matched, the jQuery object is an empty collection.

When jQuery cannot find an object or element, it returns an empty jQuery object. This empty object is created by the jQuery.fn.init() function. It does not have any properties and methods and can only store some jQuery objects.

For example, the following code will try to find an element with the id "not-exist":

var $elem = $('#not-exist');
console.log($elem);

Because the "not-exist" element does not exist, the jQuery selector cannot find the corresponding element. object, so it will return an empty jQuery object, which is []. We can determine whether the corresponding element has been found by judging the length of this object:

if ($elem.length === 0) {
    console.log('没有找到相应的元素！');
}

Of course, if we do not need to output information on the console, we can also use the following code directly:

if (!$('#not-exist').length) {
    // 代码不执行
}

If we call a method that does not exist, jQuery will throw a "TypeError: $elem.fn is undefined" error. Therefore, when using jQuery to operate DOM elements, we must first determine whether the selector matches the corresponding element to prevent errors when the element cannot be found.

In addition to using selectors to match elements, we often use some of jQuery’s convenient methods, such as $.ajax(), $.get(), etc. These methods will return a failed Promise object when the corresponding URL cannot be found.

For example, the following code will use the $.ajax() method to request a URL that does not exist:

$.ajax({
    url: 'http://example.com/404',
    success: function () {
        console.log('请求成功');
    },
    error: function () {
        console.log('请求失败');
    }
});

Because the requested URL does not exist, $.ajax() will return a failure Promise object, calls the error callback function and outputs the "request failed" message.

In general, jQuery will return an empty jQuery object or a failed Promise object when it cannot find the corresponding object or URL. When we use jQuery, we must pay attention to determine whether the selector matches the element or whether the requested URL exists to avoid unnecessary errors.

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