resize-image.js

// sourceImage can be any File or Blob
async resizeImage(sourceImage){
    let el = document.createElement("img")
    let prom = new Promise( (res, rej) => {
        el.addEventListener("load", res)
        el.addEventListener("error", rej)
    })
    el.src = URL.createObjectURL(sourceImage)
    await prom
    let width = 0;
    let height = 0;
    if(el.naturalWidth > el.naturalHeight) {
        width = Math.min(1920, el.naturalWidth);
        height = (el.naturalHeight * width) / el.naturalWidth
    } else  {
        height = Math.min(1920, el.naturalHeight);
        width = (el.naturalWidth * height) / el.naturalHeight
    }
    let canvas = new OffscreenCanvas(width, height);
    let ctx = canvas.getContext("2d")
    ctx.drawImage(el, 0, 0, width, height)
    let blob = await canvas.convertToBlob({
        type: "image/jpeg",
        quality: 0.9
    })
    URL.revokeObjectURL(el.src)
    return blob
}