js上传头像进行图片压缩

1、compress(img)  传入要压缩的图片元素，返回一个base64

例如： compress（document.getElementById("img")）

// 图片压缩
    function compress(img) {
        let canvas = document.createElement("canvas");
        let ctx = canvas.getContext('2d');
        //瓦片canvas
        let tCanvas = document.createElement("canvas");
        let tctx = tCanvas.getContext("2d");
        let initSize = img.src.length;
        let width = img.width;
        let height = img.height;

        //如果图片大于四百万像素，计算压缩比并将大小压至400万以下
        let ratio;
        if ((ratio = width * height / 4000000) > 1) {
            console.log("大于400万像素")
            ratio = Math.sqrt(ratio);
            width /= ratio;
            height /= ratio;
        } else {
            ratio = 1;
        }
        canvas.width = width;
        canvas.height = height;
        //        铺底色
        ctx.fillStyle = "#fff";
        ctx.fillRect(0, 0, canvas.width, canvas.height);
        //如果图片像素大于100万则使用瓦片绘制
        let count;
        if ((count = width * height / 1000000) > 1) {
            count = (Math.sqrt(count) + 1); //计算要分成多少块瓦片
            //            计算每块瓦片的宽和高
            let nw = (width / count);
            let nh = (height / count);
            tCanvas.width = nw;
            tCanvas.height = nh;
            for (let i = 0; i < count; i++) {
                for (let j = 0; j < count; j++) {
                    tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio, nh * ratio, 0, 0, nw, nh);
                    ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
                }
            }
        } else {
            ctx.drawImage(img, 0, 0, width, height);
        }
        //进行最小压缩
        let ndata = canvas.toDataURL('image/jpeg', 0.1);
        console.log('压缩前：' + initSize);
        console.log('压缩后：' + ndata.length);
        console.log('压缩率：' + ~~(100 * (initSize - ndata.length) / initSize) + "%");
        tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;

        return ndata;
    }

上传头像图片像素比较大，可以进行压缩之后传给后台（剪切头像使用的cropper.js，使用cropper.js实现图片剪切）

调用上面的 compress 方法进行图片压缩