JS compresses uploaded images

/**
 * @Date: 2016/11/17  0017
 * @Time: 10:14
 * @Author: lxbin
 *
 * Created with JetBrains WebStorm.
 */

/**
 * http://leonshi.com/2015/10/31/html5-canvas-image-compress-crop/
 * http://jafeney.com/2016/08/11/20160811-image-upload/
 */

/**
 * 读取文件
 * @param file 文件对象
 * @return {Promise}
 */
function readFileAsync(file) {
    return new Promise((resolve, reject) => {
        const reader = new FileReader()
        reader.onload = e => resolve(readFile.target.result)
        reader.onerror = e => reject(new Error('Could not read file'))
        reader.readAsDataURL(file)
    })
}

/**
 * 加载图片
 * @param url 图片地址
 * @return {Promise}
 */
function loadImageAsync(url) {
    return new Promise((resolve, reject) => {
        const image = new Image()
        image.onload = () => resolve(image)
        image.onerror = () => reject(new Error('Could not load image at ' + url))
        image.src = url
    })
}

/**
 * base64的图片dataUri转Blob
 * @param dataURI
 * @return {*}
 */
function dataURItoBlob(dataURI) {
    // convert base64 to raw binary data held in a string
    // doesn't handle URLEncoded DataURIs - see SO answer #6850276 for code that does this
    var byteString = atob(dataURI.split(',')[1]);

    // separate out the mime component
    var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]

    // write the bytes of the string to an ArrayBuffer
    var ab = new ArrayBuffer(byteString.length);
    var ia = new Uint8Array(ab);
    for (var i = 0; i < byteString.length; i++) {
        ia[i] = byteString.charCodeAt(i);
    }

    // write the ArrayBuffer to a blob, and you&#39;re done
    var blob = new Blob([ab], {type: mimeString});
    return blob;

    // Old code
    // var bb = new BlobBuilder();
    // bb.append(ab);
    // return bb.getBlob(mimeString);
}

/**
 * 图片转Blob
 * @param image 图片对象
 * @param quality 图片质量（0到1之间）
 * @param scale 缩放比例（0到1之间）
 * @return {Promise}
 */
function imageToBlob(image, quality, scale) {
    return new Promise((resolve, reject) => {
        try {
            let canvas = document.createElement(&#39;canvas&#39;)
            canvas.width = image.naturalWidth * scale
            canvas.height = image.naturalHeight * scale
            while (canvas.width >= 3264 || canvas.height >= 2448) {//超过这个值base64无法生成，在IOS上
                canvas.width = canvas.naturalWidth * scale
                canvas.height = canvas.naturalHeight * scale
            }

            let ctx = canvas.getContext(&#39;2d&#39;).drawImage(image, 0, 0, canvas.width, canvas.height)

            //方式一：低版本兼容性差些
            //canvas.toBlob(function (blob) {
            //    console.group(&#39;[Leo]file compress to blob&#39;)
            //    console.log(&#39;文件类型 => &#39; + file.type)
            //    console.log(&#39;文件大小 => &#39; + (file.size / 1024 / 1024).toFixed(2) + &#39;M&#39;)
            //    console.log(&#39;blob质量 => &#39; + quality)
            //    console.log(&#39;blob大小 => &#39; + (blob.size / 1024 / 1024).toFixed(2) + &#39;M&#39;)
            //    console.groupEnd()
            //    resolve(blob)
            //}, &#39;image/jpeg&#39;, quality)

            //方式二：
            var base64 = canvas.toDataURL(&#39;image/jpeg&#39;, quality);
            var blob = dataURItoBlob(base64);
            console.group(&#39;[Leo]file compress to blob&#39;)
            console.log(&#39;文件类型 => &#39; + file.type)
            console.log(&#39;文件大小 => &#39; + (file.size / 1024 / 1024).toFixed(2) + &#39;M&#39;)
            console.log(&#39;blob质量 => &#39; + quality)
            console.log(&#39;blob大小 => &#39; + (blob.size / 1024 / 1024).toFixed(2) + &#39;M&#39;)
            console.groupEnd()
            resolve(blob);
        } catch (e) {
            reject(new Error("Image could not convert to blob :" + e))
        }
    })
}

/**
 * Ajax上传
 * @param uri 上传的Action地址
 * @param file 文件对象
 * @return {Promise}
 */
function uploadFile(uri, file) {
    return new Promise((resolve, reject) => {
        let xhr = new XMLHttpRequest()
        if (xhr.upload) {
            xhr.upload.addEventListener("progress", (e) => {// 处理上传进度
                if (e.lengthComputable) {
                    let percent = (e.loaded / e.total * 100).toFixed(2) + &#39;%&#39;
                    console.log("上传中(" + percent + ")");
                    //TODO：反馈到DOM里显示
                } else {
                    console.log(&#39;unable to compute&#39;);
                }
            }, false)
        }
        xhr.onreadystatechange = (e) => {// 文件上传成功或是失败
            if (xhr.readyState === 4) {
                if (xhr.status === 200) {
                    resolve(xhr.responseText)// 上传成功
                } else {
                    reject(xhr.responseText)// 上传出错处理
                }
            }
        }
        xhr.open("POST", uri, true)// 开始上传
        let form = new FormData()
        form.append("filedata", file)
        xhr.send(form)
    })
}

/**
 * 上传文件
 * @param file 文件对象
 * @param quality 图片质量（0到1之间）
 * @param scale 缩放比例（0到1之间）
 */
export default async function fileUpload(file, quality, scale) {
    try {
        let fileUrl = await readFileAsync(file)
        let image = await loadImageAsync(fileUrl)
        let blob = await imageToBlob(image, quality, scale)
        let upload = await uploadFile(blob)
        return upload
    } catch (e) {
        console.log(&#39;file upload failed&#39;)
    }
}