尽管我添加了新的字符串项，为什么数组的长度为零？

Question

我想从Excel中提取数据到数组。在这里，“数据”采用JSON数组的形式。我提取了名为“ ASIN”的字符串，然后将它们添加到“ assinList”数组中。当我控制“ assinList”的长度时，结果为零。为什么会这样？我不明白怎么了。

function ExcelResults(url){
    var asinList = new Array(25);
    var jo = 0;
    var constantURL = 'url';
    var oReq = new XMLHttpRequest();
    oReq.open("GET", url, true);
    oReq.responseType = "arraybuffer";
    oReq.onload = function(e) {
        var arraybuffer = oReq.response;

        var data = new Uint8Array(arraybuffer);
        var arr = new Array();
        for(var i = 0; i != data.length; ++i) arr[i] = String.fromCharCode(data[i]);
        var bstr = arr.join("");

        var workbook = XLSX.read(bstr, {type:"binary"});
        var first_sheet_name = workbook.SheetNames[0];
        var worksheet = workbook.Sheets[first_sheet_name];

        var datass = XLSX.utils.sheet_to_json(worksheet,{raw:true});
        datass.forEach(function(el){
            asinList[jo] = el.ASIN;
            jo++;
        });
    }
    oReq.send();
    asinList.push('reddit')
    return asinList;
}