问循环内部的const用法，为什么是这种行为？

EN

const saveDocument = co.wrap(function *(documentData, user, locale) {
    var now = moment();
    var creationDateLongString = now.format("YYYYMMDDHHmmss");
    var creationDateShortString = now.format("YYYYMMDD");

    var outputChildFolder = documentData.code + '_' + creationDateLongString + '_' + documentCounter;
    var outputFolder = config.files.incomingDocumentsDir + '/' + outputChildFolder;

    ++documentCounter;
    yield fs.mkdir(outputFolder)

    var xmlFileName = documentData.code + "-" + creationDateLongString + ".xml";
    var pdfFileName = documentData.code + "-" + creationDateLongString + ".pdf";

    const pages = [];

    for(var index=0;index < documentData.pages.length; ++index) {
        const page = documentData.pages[index];
        var data = new Buffer(page, "base64");
        var dataEncoding = imageType(data).mime === "image/png" ? "png" : "jpg";
        var fileName = "page" + index + "." + dataEncoding;
        var targetFilePath = outputFolder + "/" + fileName
        yield fs.writeFile(targetFilePath,data);
        pages.push(fileName);
    }
...
}