jquery determines whether file has attachments

jQuery is a very popular JavaScript library that has been widely used in Web development. It can provide developers with a wealth of tools and functions, simplifying the work of processing DOM objects, handling events, and writing dynamic effects. In web development, file upload is one of the common functions. Before uploading a file, it is often necessary to determine whether the user has selected the file. This article will introduce how to use jQuery to determine whether the selected file has an attachment.

1. HTML

First, we need to create an input tag for file upload, as shown below:

<form>
  <input type="file" id="upload_file">
  <button id="upload_btn">上传</button>
</form>

whereinput type= The "file" tag represents the input box for file upload, and id="upload_file" represents its ID, which can facilitate subsequent use of jQuery to obtain the element. The button tag is a submit button.

2. jQuery code

Next, we need to write code in jQuery to determine whether the user has selected the uploaded file and perform corresponding operations. The specific operation is as follows:

$(document).ready(function() {
  $("#upload_btn").click(function() {
    var file = $("#upload_file").prop("files")[0]; //获取上传文件
    if (typeof file === "undefined") { //如果没有选择文件
      alert("请选择上传的文件");
    } else { //否则已选择文件，进行文件上传的其他操作
      // ...
    }
  });
});

After the document is loaded, we use the $(document).ready() function to wrap the entire code block to ensure that all elements in the document have been loaded. complete. We then specified the upload button using $("#upload_btn") and added a click event to the element. After that, we use $("#upload_file").prop("files")[0] to get the file selected by the user. Among them, $("#upload_file") means to get the element with ID upload_file, .prop() is to get the attributes of the element, wherefilesAttribute refers to the list of files selected by the user. Since only one file can be uploaded at a time, we only get the first element of the list, which is [0].

At this time, we use typeof to determine whether the element is undefined. If it is undefined, it means that the user has not selected a file, and we will use the alert() function that comes with JavaScript to prompt the user to select a file. Otherwise, it means that the file is selected, and we can use other code to upload the file.

3. Picture preview

In actual development, we often need to be able to preview the selected file in real time after the user chooses to upload the file. Here, we take uploading pictures as an example to introduce how to use jQuery to implement the picture preview function.

$(document).ready(function() {
  $("#upload_file").change(function() { //监听文件选择事件
    var file = $("#upload_file").prop("files")[0];
    if (typeof file === "undefined") return; //如果没有选择文件，则返回
    if (!/^image/(jpeg|png|gif)$/.test(file.type)) { //如果选择的不是图片格式
      alert("请选择jpg、png或gif格式的图片");
      return;
    }

    var reader = new FileReader();
    reader.onload = function(e) { //文件加载完毕后，执行回调函数
      $("#preview_img").attr("src", e.target.result); //使用jQuery修改图片的src属性
    }
    reader.readAsDataURL(file); //读取文件
  });
});

We added a change event to the input type="file" element of file upload to listen for the file selection event. When the user selects a file, we first obtain the file using the same method as above, and use regular expressions to detect whether the file is in image format. If not, use the alert() function to prompt the user to select a file in image format.

For the selected image file, we need to use the FileReader() object to read its content. This is the file reading API provided by HTML5. After the file is read, the callback function is automatically triggered. In the callback function, we use $("#preview_img").attr("src", e.target.result) to change the src attribute of the image to achieve preview Function.

4. Conclusion

As web applications become more and more complex, it will become more and more common to use jQuery to operate files. The method introduced in this article is only a simple example of file operation, and I hope it can inspire readers. In actual applications, issues such as file size, upload progress, and multiple file uploads also need to be taken into consideration, and developers need to handle them according to the actual situation.

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