How to implement image cropping and uploading function in JavaScript?

JavaScript How to realize image cropping and uploading function?

In web development, we often encounter the need for users to upload and crop images, such as avatar uploading, image editing, etc. JavaScript provides a wealth of APIs and functions that can help us implement such functions. This article will introduce how to use JavaScript to implement image cropping and uploading functions, and provide specific code examples.

First, we need to add an element for displaying pictures in the HTML file, such as an img tag:

<img id="previewImage" src="#" alt="Preview Image" />

Next, we need to add an input element for file upload, for Let users select images to upload. At the same time, we also need to add a button to trigger the image cropping operation:

<input type="file" id="fileInput" />
<button id="cropButton">裁剪并上传</button>

In JavaScript code, we can use the FileReader object to read the image file selected by the user. Once the user selects a picture, by listening to the change event, we can obtain the image file object selected by the user:

const fileInput = document.getElementById("fileInput");
const imgPreview = document.getElementById("previewImage");
fileInput.addEventListener("change", function(event) {
  const file = event.target.files[0];
  const reader = new FileReader();
  reader.onload = function(e) {
    imgPreview.src = e.target.result;
  };
  reader.readAsDataURL(file);
});

The above code reads the image file selected by the user into Base64 format data , and displayed on the page for users to preview.

Next, we need to add the function of image cropping. Here we can use the third-party library cropper.js to achieve this. First, we need to introduce the cropper.js library:

<script src="cropper.js"></script>

Next, after the user selects the image, we can initialize a Cropper object and pass in the image to be cropped Picture elements:

let cropper;
fileInput.addEventListener("change", function(event) {
  const file = event.target.files[0];
  const reader = new FileReader();
  reader.onload = function(e) {
    imgPreview.src = e.target.result;
    if (cropper) {
      cropper.destroy();
    }
    cropper = new Cropper(imgPreview, {
      aspectRatio: 1, // 裁剪框的宽高比例
      viewMode: 1, // 显示裁剪框的模式
      dragMode: 'move', // 裁剪框的拖拽模式
      cropBoxResizable: false // 裁剪框是否可以改变大小
    });
  };
  reader.readAsDataURL(file);
});

Now, users can select the cropped area by dragging the mouse. When the user clicks the "Crop and Upload" button, we can obtain the cropped image data and upload it.

const cropButton = document.getElementById("cropButton");
cropButton.addEventListener("click", function() {
  const canvas = cropper.getCroppedCanvas();
  // 将裁剪后的图片数据转换为Blob对象
  canvas.toBlob(function(blob) {
    // 创建FormData对象，用于文件上传
    const formData = new FormData();
    formData.append("file", blob, "image.jpg");
    // 发送请求，上传文件
    fetch("upload.php", {
      method: "POST",
      body: formData
    })
    .then(response => response.text())
    .then(data => {
      console.log(data); // 上传成功后的处理逻辑
    })
    .catch(error => {
      console.error(error); // 上传出错的处理逻辑
    });
  });
});

In the above code, we use the cropper.getCroppedCanvas() method to obtain the cropped image data and convert it into a Blob object. Then, we create a FormData object and add the cropped image Blob data to the form. Finally, send a request with image data through the fetch function to upload the image to the server.

The above are the detailed steps and code examples on how to use JavaScript to implement image cropping and uploading functions. Hope this helps!

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