javascript压缩上传图片

export function compressPicture(file) {

    return new Promise((resolve, reject) => {
        let width, height;
        let MAX_WH = 800;
        let image = new Image()

        image.onload = () => {
            if (image.height > MAX_WH) {
                // 宽度等比例缩放 *=
                image.width *= MAX_WH / image.height;
                image.height = MAX_WH;
            }
            if (image.width > MAX_WH) {
                // 宽度等比例缩放 *=
                image.height *= MAX_WH / image.width;
                image.width = MAX_WH;
            }

            let canvas = document.createElement('canvas'); // 创建Canvas对象(画布)
            let context = canvas.getContext('2d');

            canvas.width = width = image.width;
            canvas.height = height = image.height;

            context.clearRect(0, 0, canvas.width, canvas.height);
            context.drawImage(image, 0, 0, image.width, image.height);

            const base64Url = canvas.toDataURL(file.type, 0.3);
            resolve(base64Url)
        }
        image.onerror = () => {
            reject()
        }

        image.src = file.url;
    })
}