js read.readAsDataURL base64 預覽並壓縮圖片

    $(".img-btn").click(function(e){
        if(e.target.nodeName == 'INPUT'){
            $('input').change(function(){
                var img = $(this).parent().find('img')
                img.addClass('imgs');
                var file = e.target.files[0]
                var read = new FileReader()
                if(file == null){
                    return
                }
                $(this).parent().find('span').show()

                read.readAsDataURL(file)
                read.onload = function(e){
                    var src = img.attr('src')
                    suofang(e.target.result,2, function(base64Codes){
                        img.attr('src',  base64Codes)     //壓縮後的圖片
                    });
                }
            })
        }
    })

function   suofang(base64, bili,callback) {
    //處理縮放，轉格式
    var _img = new Image();
    _img.src = base64;
    _img.onload = function () {
        var _canvas = document.createElement("canvas");
        var w = this.width / bili;
        var h = this.height / bili;
        _canvas.setAttribute("width", w);
        _canvas.setAttribute("height", h);
        _canvas.getContext("2d").drawImage(this, 0, 0, w, h);
        var base64 = _canvas.toDataURL("image/jpeg",0.5);         0.5  數字越小越不清析
        callback(base64);    //這個只能用  callback 方法獲取
    }
}