To post image with fetch and JavaScript, we call fetch with the body set to the form data object with the file to upload.

For instance, we write

const fileInput = document.querySelector("#your-file-input");
const formData = new FormData();

formData.append("file", fileInput.files[0]);

const options = {
  method: "POST",
  body: formData,
};

fetch("your-upload-url", options);

to select the file input with querySelector.

Then we create a FormData instance and put the selected file there with append.

fileInput.files[0] has the first selected file.

Then we call fetch with the upload URL and options, which has body set to formData.