Once I have a base64 url how can I change the filter for brightness and contrast?

If I have an image in base64 ex.

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAABDAAwfqNRk/rjcYX+PxrV/wtWqwJSTlboMgAAAABJRU5ErkJggg==

What's the fastest way to change a filter (ex. brightness, contrast) programmatically and without creating any img or canvas tag in the view? Then once changed, convert it back to a base64 url?

You can do this by putting the image onto a HTML canvas with filters applied, then turning that canvas back into a base64 URL.

Since you never add the canvas to the document, it will never be seen.

This is how you would do it asynchronously:
This takes about 1-10ms

function applyFiltersToImage(imageURL, callBack) {
    // load the image url into an image object
    const image = new Image();
    image.src = imageURL;

    const canvas = document.createElement("canvas");
    const ctx = canvas.getContext('2d');
    image.onload = () => {        
        canvas.width = image.width;
        canvas.height = image.height;

        // apply css filters here
        ctx.filter = 'brightness(0.5) contrast(2)';

        ctx.drawImage(image, 0, 0, canvas.width, canvas.height);
        
        // turn canvas back into a base64 image URL
        callBack(canvas.toDataURL("image/png"));
    }
}

You could do it synchronously if you already have the Image instance loaded.

This is how you will do it synchronously:

function applyFiltersToImageSync(imageObject) {
    const canvas = document.createElement("canvas");
    const ctx = canvas.getContext('2d');    
    canvas.width = imageObject.width;
    canvas.height = imageObject.height;

    // apply css filters here
    ctx.filter = 'brightness(0.5) contrast(2)';

    ctx.drawImage(imageObject, 0, 0, canvas.width, canvas.height);
    //turn canvas back into a base64 image
    return canvas.toDataURL("image/png");
}