如果不存在，jQuery追加 [英] jQuery append if doesn't exist

问题描述

我想附加一个 div 只有它还没有。我正在尝试这个但它不起作用：

I would like to append a div only if it's not already there. I am trying with this but it doesn't work:

$('#method_id').on('change', function (e) {

    if ($(this).find("option:selected").data('method-column-id') != 1) {
        if ($('#column_message').next('div').length)
            $('#column_message')
                    .append('<div id="warning_message" class="alert alert-danger"><strong>Warning!</strong> Installed column and method do not match</div>');
    } else {
        $('.form-group').find('#column_message>.alert').remove();
    }
});

如果我删除第二个if子句，每当我选择一个通过的选项时它会被追加第一个if子句

And if I remove the second if-clause, it gets appended every time I select an option which passes first if-clause

这是HTML

<!-- Warning message -->
<div class="form-group">
    <label for="group" class="col-md-2 control-label"></label>
    <div id="column_message" class="col-md-6">
    </div>
</div>

推荐答案

为了检查元素是否存在，你需要找到一个只匹配该元素的选择器，并检查jQuery对象的长度是否等于0，如下所示：

In order to check if an element exists, you need to find a selector that matches only that element and check if the length of the jQuery object is equal to 0 or not, like so:

if ($('#unique-selector').length === 0) {
    // code to run if it isn't there
}
else {
    // code to run if it is there
}

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