Dont understand how write file to blob

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There are two versions of the code, in the first version, it is the picture that is written to the blob (which I am looking for and dropping it into the input)

JavaScript

let File = document.querySelector('input').files[0]
let blobUpload = new Blob([File], {type: 'image/jpg'},)

In the second option, the picture is not written to the blob

JavaScript

let File = new Image()
File.src = "car1.jpg"
let blobUpload = new Blob([File], {type: 'image/jpg'},)

What I have tried:

As I understand it, it does not see img as a file
The question is, how do I record the picture manually - without dragging the file?

Posted 16-Dec-22 4:07am

jonathan rogers Dec2022

Updated 16-Dec-22 4:53am

Richard Deeming

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Richard Deeming · Accepted Answer · 2022-12-16T04:53:00

Creating a new Image and setting its src will not block the script and wait for the image to finish downloading.

You need to wait for the remote image to load, then use a canvas to convert it to Blob:
https://javascript.info/blob#image-to-blob[^]

Create a couple of helper methods:

C#

const loadImageAsync = (url) => new Promise((resolve, reject) => {
    const img = new Image();
    img.onload = () => resolve(img);
    img.onerror = () => reject(`Unable to load image ${url}.`);
    img.setAttribute("crossorigin", "anonymous");
    img.src = url;
});

const imageToBlobAsync = (img, type = "image/png") => new Promise((resolve, reject) => {
    const canvas = document.createElement("canvas");
    canvas.width = img.naturalWidth;
    canvas.height = img.naturalHeight;
    
    const context = canvas.getContext("2d");
    context.drawImage(img, 0, 0);
    
    canvas.toBlob(resolve, type);
});

You can then either pass a callback to the methods:

JavaScript

loadImageAsync("car1.jpg").then(img => imageToBlobAsync(img, "image/jpg")).then(blobUpload => {
    ...
});

Or use an async function[^]:

JavaScript

const uploadImage = async () => {
    const img = await loadImageAsync("car1.jpg");
    const blobUpload = await imageToBlobAsync(img, "image/jpg");
    ...
};