项目中上传头像base64图片太大导致上传时间过长，或者后台奔溃问题，所以前端压缩base64大小解决这个问题
原理使用
canvas进行图片压缩参数说明：
base64String：需要压缩的base64图片
w: 图片的原始图片大小
quality: 0.7 // 值越小，所绘制出的图像越模糊
示例：
let base64Img = pressImg(file.content, 100, 0.7);
base64Img就是压缩完成后的base64图片
compressImg(base64String, w, quality) {var getMimeType = function(urlData) {var arr = urlData.split(",");var mime = arr[0].match(/:(.*?);/)[1];return mime;};var newImage = new Image();var imgWidth, imgHeight;var promise = new Promise(resolve => (newImage.onload = resolve));newImage.src = base64String;return promise.then(() => {imgWidth = newImage.width;imgHeight = newImage.height;var canvas = document.createElement("canvas");var ctx = canvas.getContext("2d");if (Math.max(imgWidth, imgHeight) > w) {if (imgWidth > imgHeight) {canvas.width = w;canvas.height = (w * imgHeight) / imgWidth;} else {canvas.height = w;canvas.width = (w * imgWidth) / imgHeight;}} else {canvas.width = imgWidth;canvas.height = imgHeight;}ctx.clearRect(0, 0, canvas.width, canvas.height);ctx.drawImage(newImage, 0, 0, canvas.width, canvas.height);var base64 = canvas.toDataURL(getMimeType(base64String),quality);console.log(base64);return base64;});}
