我试图从foreach循环中的回调函数中获取数据，但它似乎无法正常工作，因为循环在调用回调函数之前结束。以下是我的功能：

function convertImgToBase64URL(url, callback, outputFormat) {
    var img = new Image();
    img.crossOrigin = 'Anonymous';
    img.onload = function () {
        var canvas = document.createElement('CANVAS'),
            ctx = canvas.getContext('2d'), dataURL;
        canvas.height = img.height;
        canvas.width = img.width;
        ctx.drawImage(img, 0, 0);
        dataURL = canvas.toDataURL(outputFormat);
        callback(dataURL);
        canvas = null;
    };
    img.src = url;
}

我调用这个函数的代码如下：

angular.forEach(alarms, function (alarm) {
                    convertImgToBase64URL(alarm.photo, function (base64Img) {
                        alarmInfo.push({
                            severity: alarm.payload.severity,
                            from: alarm.name,
                            location: alarm.location,
                            alarmTime: alarm.time,
                            photo: base64Img,
                            contextId: alarm.id
                        });
                        $log.info(base64Img);
                     });

            });

有没有办法让循环等待回调函数完成，然后继续循环？