因为图片紧缩中运用了大批异步操纵，这里运用es7 async 语法处置惩罚异步题目

基本思路是:先将file数据读取为DataUrl范例的数据，再将DataUrl绘制到canvas，

经由过程canvas的apitoDataURL转换为base64数据并紧缩数据，末了再将base64数据转换

为blob数据范例，再上传到服务器。全部流程就完成了。下面是详细的完成体式格局。

1.读取file转换为DataUrl

async function fileLoad(file){
        return new Promise((resolve)=>{
            let ready = new FileReader();
            /*最先读取指定的Blob对象或File对象中的内容. 当读取操纵完成时,
            readyState属性的值会成为DONE,假如设置了onloadend事宜处置惩罚顺序,
            则调用之.同时,result属性中将包括一个data: URL花样的字符串以表
            示所读取文件的内容.*/
            ready.readAsDataURL(file);
            ready.onload = function () {
                resolve(this.result);
            }
        })
    }

2.将数据绘制到canvas上，紧缩数据

async function canvasDataURL(path, obj) {
        return new Promise((resolve)=>{
            let img = new Image();
            img.src = path;
            img.onload = function () {
                let that = this;
                // 默许按比例紧缩
                let w = that.width,
                    h = that.height,
                    scale = w / h;
                w = obj.width || w;
                h = obj.height || (w / scale);
                let quality = 0.7;  // 默许图片质量为0.7
                //天生canvas
                let canvas = document.createElement('canvas');
                let ctx = canvas.getContext('2d');
                // 建立属性节点
                let anw = document.createAttribute("width");
                anw.nodeValue = w;
                let anh = document.createAttribute("height");
                anh.nodeValue = h;
                canvas.setAttributeNode(anw);
                canvas.setAttributeNode(anh);
                ctx.drawImage(that, 0, 0, w, h);
                // 图象质量
                if (obj.quality && obj.quality <= 1 && obj.quality > 0) {
                    quality = obj.quality;
                }
                // quality值越小，所绘制出的图象越隐约
                let base64 = canvas.toDataURL('image/jpeg', quality);
                // 回调函数返回base64的值
                resolve(base64);
            }
        });
    }

3.将bas64转换为blob

async function convertBase64UrlToBlob(urlData) {
        return new Promise((resolve)=>{
            let arr = urlData.split(','), mime = arr[0].match(/:(.*?);/)[1],
                bstr = atob(arr[1]), n = bstr.length, u8arr = new Uint8Array(n);
            while (n--) {
                u8arr[n] = bstr.charCodeAt(n);
            }
            let blob = new Blob([u8arr], {type: mime});
            resolve(blob);
        })
    }

4.末了完成紧缩要领

/**
     * 三个参数
     * file：一个是文件(范例是图片花样)，
     * w：设置对象
     * photoCompress()
     */
async function photoCompress(file, w) {
        let re = await fileLoad(file);
        let base64 = await canvasDataURL(re, w);
        let blob = await convertBase64UrlToBlob(base64);
        return new Promise((resolve)=>{
            resolve(blob);
        })
}

5.运用要领

async function upload(file) {
    let form = new FormData();
    let fileData = await photoCompress(file, {
        quality: 0.5
  });
 form.append('pictrue',fileData,file.name);
 // 上传form
}