壓縮base64圖片大小

項目中上傳頭像base64圖片太大導致上傳時間過長，或者後臺奔潰問題，所以前端壓縮base64大小解決這個問題
原理使用canvas進行圖片壓縮
參數說明：
base64String：需要壓縮的base64圖片
w: 圖片的原始圖片大小
quality: 0.7 // 值越小，所繪製出的圖像越模糊

示例：let base64Img = this.compressImg(file.content, 100, 0.7);
base64Img就是壓縮完成後的base64圖片

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;
			});
		}