当我在nodejs中创建api并尝试将mongoose返回计数推送到新创建的数组时，它不会等待forEach并执行json.res()并给出空响应。当我使用setTimeout（）时，它会给出正确的结果。
let newcategories = [];
let service = 0;
const categories = await Category.find({}, '_id name');
categories.forEach(async (category) => {
service = await Service.count({category: category});
newcategories.push({ count:service });
console.log('newcategories is -- ', newcategories);
}); /* while executing this forEach it's not wait and execute res.json..*/
console.log('result --- ',result);
console.log('out newcategories is -- ', newcategories);
res.json({status: 200, data: newcategories});
您需要使用map而不是forEach来收集等待并等待它们完成。编辑:或者您可以使用
null
const categories = ['a', 'b', 'c'];
function getNextCategory(oldCategory) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(String.fromCharCode(oldCategory.charCodeAt(0)+1));
}, 1000);
});
}
async function blah() {
const categoryPromises = categories.map(getNextCategory);
const nextCategories = await Promise.all(categoryPromises);
console.log(nextCategories);
}
blah();
async function blah2() {
const nextCategories = [];
for (const category of categories) {
nextCategories.push(await getNextCategory(category));
};
console.log(nextCategories);
}
blah2();
因此，问题在于
在
您可以选择将语句转换为
for..of循环如下所示
for (let category of categories) {
service = await Service.count({category: category});
newcategories.push({ count:service });
console.log('newcategories is -- ', newcategories);
}
地图版本如下所示
await Promise.all( categories.map(async (category) => {
service = await Service.count({category: category});
newcategories.push({ count:service });
console.log('newcategories is -- ', newcategories);
}));
第二个版本之所以能工作，是因为只有当所有承诺完成后，promise.all才会解析，并且映射将为每个类别返回一个可能未解析的承诺
