Hướng dẫn dùng versus用法 JavaScript

Accessing selected file[s]

Consider this HTML:

The File API makes it possible to access a FileList containing File objects representing the files selected by the user.

The multiple attribute on the input element allows the user to select multiple files.

Accessing the first selected file using a classical DOM selector:

const selectedFile = document.getElementById['input'].files[0];

Accessing selected file[s] on a change event

It is also possible [but not mandatory] to access the FileList through the change event. You need to use EventTarget.addEventListener[] to add the change event listener, like this:

const inputElement = document.getElementById["input"];
inputElement.addEventListener["change", handleFiles, false];
function handleFiles[] {
  const fileList = this.files; /* now you can work with the file list */
}

Getting information about selected file[s]

The FileList object provided by the DOM lists all of the files selected by the user, each specified as a File object. You can determine how many files the user selected by checking the value of the file list's length attribute:

const numFiles = fileList.length;

Individual File objects can be retrieved by accessing the list as an array:

for [let i = 0, numFiles = fileList.length; i DOCTYPE html>

  
    
    File[s] size
  

  
    
      
        
        Selected files:
        0;
        Total size:
        0
      
      
    

    
      const uploadInput = document.getElementById["uploadInput"];
      uploadInput.addEventListener[
        "change",
        [] => {
          // Calculate total size
          let numberOfBytes = 0;
          for [const file of uploadInput.files] {
            numberOfBytes += file.size;
          }

          // Approximate to the closest prefixed unit
          const units = [
            "B",
            "KiB",
            "MiB",
            "GiB",
            "TiB",
            "PiB",
            "EiB",
            "ZiB",
            "YiB",
          ];
          const exponent = Math.min[
            Math.floor[Math.log[numberOfBytes] / Math.log[1024]],
            units.length - 1
          ];
          const approx = numberOfBytes / 1024 ** exponent;
          const output =
            exponent === 0
              ? `${numberOfBytes} bytes`
              : `${approx.toFixed[3]} ${
                  units[exponent]
                } [${numberOfBytes} bytes]`;

          document.getElementById["fileNum"].textContent = fileList.length;
          document.getElementById["fileSize"].textContent = output;
        },
        false
      ];
    
  

Selecting files using drag and drop

You can also let the user drag and drop files into your web application.

The first step is to establish a drop zone. Exactly what part of your content will accept drops may vary depending on the design of your application, but making an element receive drop events is easy:

let dropbox;

dropbox = document.getElementById["dropbox"];
dropbox.addEventListener["dragenter", dragenter, false];
dropbox.addEventListener["dragover", dragover, false];
dropbox.addEventListener["drop", drop, false];

In this example, we're turning the element with the ID dropbox into our drop zone. This is done by adding listeners for the dragenter, dragover, and drop events.

We don't actually need to do anything with the dragenter and dragover events in our case, so these functions are both simple. They just stop propagation of the event and prevent the default action from occurring:

function dragenter[e] {
  e.stopPropagation[];
  e.preventDefault[];
}

function dragover[e] {
  e.stopPropagation[];
  e.preventDefault[];
}

The real magic happens in the drop[] function:

function drop[e] {
  e.stopPropagation[];
  e.preventDefault[];

  const dt = e.dataTransfer;
  const files = dt.files;

  handleFiles[files];
}

Here, we retrieve the dataTransfer field from the event, pull the file list out of it, and then pass that to handleFiles[]. From this point on, handling the files is the same whether the user used the input element or drag and drop.

Example: Showing thumbnails of user-selected images

Let's say you're developing the next great photo-sharing website and want to use HTML to display thumbnail previews of images before the user actually uploads them. You can establish your input element or drop zone as discussed previously and have them call a function such as the handleFiles[] function below.

function handleFiles[files] {
  for [let i = 0; i  { aImg.src = e.target.result; };
    reader.readAsDataURL[file];
  }
}

Here our loop handling the user-selected files looks at each file's type attribute to see if its MIME type begins with the string "image/"]. For each file that is an image, we create a new img element. CSS can be used to establish any pretty borders or shadows and to specify the size of the image, so that doesn't need to be done here.

Each image has the CSS class obj added to it, making it easy to find in the DOM tree. We also add a file attribute to each image specifying the File for the image; this will let us fetch the images for actual upload later. We use Node.appendChild[] to add the new thumbnail to the preview area of our document.

Next, we establish the FileReader to handle asynchronously loading the image and attaching it to the img element. After creating the new FileReader object, we set up its onload function and then call readAsDataURL[] to start the read operation in the background. When the entire contents of the image file are loaded, they are converted into a data: URL which is passed to the onload callback. Our implementation of this routine sets the img element's src attribute to the loaded image which results in the image appearing in the thumbnail on the user's screen.

Using object URLs

The DOM URL.createObjectURL[] and URL.revokeObjectURL[] methods let you create simple URL strings that can be used to reference any data that can be referred to using a DOM File object, including local files on the user's computer.

When you have a File object you'd like to reference by URL from HTML, you can create an object URL for it like this:

const objectURL = window.URL.createObjectURL[fileObj];

The object URL is a string identifying the File object. Each time you call URL.createObjectURL[], a unique object URL is created even if you've created an object URL for that file already. Each of these must be released. While they are released automatically when the document is unloaded, if your page uses them dynamically you should release them explicitly by calling URL.revokeObjectURL[]:

URL.revokeObjectURL[objectURL];

Example: Using object URLs to display images

This example uses object URLs to display image thumbnails. In addition, it displays other file information including their names and sizes.

The HTML that presents the interface looks like this:

Select some files

  No files selected!

This establishes our file element as well as a link that invokes the file picker [since we keep the file input hidden to prevent that less-than-attractive user interface from being displayed]. This is explained in the section Using hidden file input elements using the click[] method, as is the method that invokes the file picker.

The handleFiles[] method follows:

const fileSelect = document.getElementById["fileSelect"],
    fileElem = document.getElementById["fileElem"],
    fileList = document.getElementById["fileList"];

fileSelect.addEventListener["click", [e] => {
  if [fileElem] {
    fileElem.click[];
  }
  e.preventDefault[]; // prevent navigation to "#"
}, false];

fileElem.addEventListener["change", handleFiles, false];

function handleFiles[] {
  if [!this.files.length] {
    fileList.innerHTML = "
No files selected!
";
  } else {
    fileList.innerHTML = "";
    const list = document.createElement["ul"];
    fileList.appendChild[list];
    for [let i = 0; i  {
        URL.revokeObjectURL[this.src];
      }
      li.appendChild[img];
      const info = document.createElement["span"];
      info.innerHTML = `${this.files[i].name}: ${this.files[i].size} bytes`;
      li.appendChild[info];
    }
  }
}

This starts by fetching the URL of the

Example: Uploading a user-selected file

Another thing you might want to do is let the user upload the selected file or files [such as the images selected using the previous example] to a server. This can be done asynchronously very easily.

Creating the upload tasks

Continuing with the code that built the thumbnails in the previous example, recall that every thumbnail image is in the CSS class obj with the corresponding File attached in a file attribute. This allows us to select all of the images the user has chosen for uploading using Document.querySelectorAll[], like this:

function sendFiles[] {
  const imgs = document.querySelectorAll[".obj"];

  for [let i = 0; i