How Can I Display a File Preview Before Uploading It in a Web Application?

Displaying a File Preview Before Upload

To enhance the user experience when uploading files, it is often desirable to provide a preview of the image before the upload process begins. By leveraging native browser capabilities, you can effortlessly achieve this without resorting to asynchronous file uploads.

To preview an image before uploading it, follow these steps:

1. Create an Input element: Create an element with the accept attribute set to "image/*". This allows users to select image files.
2. Capture File Selection: Use an event listener on the element to capture the file selection. When a file is selected, the event handler will be triggered.
3. Generate Object URL: Inside the event handler, access the selected file using imgInp.files[0]. To display the image, generate a temporary URL using URL.createObjectURL(file).
4. Display Image: Assign the generated URL to the src attribute of an element on your page. This will render the image as a preview.

Here's an example code snippet demonstrating how to achieve this:

const imgInp = document.getElementById("imgInp");

imgInp.onchange = (evt) => {
  const [file] = imgInp.files;
  if (file) {
    const blah = document.getElementById("blah");
    blah.src = URL.createObjectURL(file);
  }
};

<form runat="server">
  <input accept="image/*" type="file">

By implementing this method, you can grant your users the ability to visualize their selected image before committing to the upload process, enhancing the user-friendliness of your web application.

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