The decode() method of the HTMLImageElement interface returns a Promise that resolves once the image is decoded and it is safe to append it to the DOM.

This can be used to initiate loading of the image prior to attaching it to an element in the DOM (or adding it to the DOM as a new element), so that the image can be rendered immediately upon being added to the DOM. This, in turn, prevents the rendering of the next frame after adding the image to the DOM from causing a delay while the image loads.

None.

A Promise that fulfills with undefined once the image data is ready to be used.

EncodingError

A DOMException indicating that an error occurred while decoding the image.

One potential use case for decode(): when loading very large images (for example, in an online photo album), you can present a low resolution thumbnail image initially and then replace that image with the full-resolution image by instantiating a new HTMLImageElement, setting its source to the full-resolution image's URL, then using decode() to get a promise which is resolved once the full-resolution image is ready for use. At that time, you can then replace the low-resolution image with the full-resolution one that's now available.

The following example shows how to use the decode() method to control when an image is appended to the DOM.

const img = new Image();
img.src = "nebula.jpg";
img
  .decode()
  .then(() => {
    document.body.appendChild(img);
  })
  .catch((encodingError) => {
    // Do something with the error.
  });

In the below example, you'll likely get an empty image shown on the page as the image is downloaded:

const img = new Image();
img.src = "img/logo.png";
document.body.appendChild(img);

Using decode() will delay inserting the image into the DOM until it is fully downloaded and decoded, thereby avoiding the empty image problem:

async function getImage() {
  const img = new Image();
  img.src = "img/logo.png";
  await img.decode();
  document.body.appendChild(img);
  const p = document.createElement("p");
  p.textContent = "Image is fully loaded!";
  document.body.appendChild(p);
}

This is particularly useful if you're dynamically swapping an existing image for a new one, and also prevents unrelated paints outside of this code from being held up while the image is decoding.

• What does the image decoding attribute actually do? on tunetheweb.com (2023)
• The HTMLImageElement.decoding property