我不需要从承诺中控制台日志我需要存储数据

Question

// get the file in Base64
function getBase64(file) {
    return new Promise((resolve, reject) => {
        const reader = new FileReader();
        reader.readAsDataURL(file);
        reader.onload = () => resolve(reader.result);
        reader.onerror = error => reject(error);
    });
}



var storeInTheGlobalVariable = '';



var ImageBase64 = getBase64(bottomBannerImage).then(function(data){
    storeInTheGlobalVariable = data; // not woking -> undefine
    console.log(data) // working fine
});

我不想管理日志。我要诺言解决数据。我如何在then（）方法之外获取它