javascript - 如何将 "transfer"变量传递到 Node Express 路由器中的 Promise 链？

Question

我试图在 promise 链中进行两个单独的数据库调用，尽管出于测试目的，第一个调用被传递的简单字符串替换。

我的问题是我无法在第二个 promise 中访问变量 msg(我尝试设置 context.foo = msg。

router.route("/")
.get(function(request, response) {

    var session = request.session;

    return new Promise(function(resolve, reject) {

        resolve("h!");

    }).then(function(msg){

        return new Promise(function(resolve, reject) {

            Snippet.find({}, function(error, data) {
                let context = {
                    snippets: data.map(function(snippet) {
                        return {
                            name: snippet.name,
                            snippet: snippet.snippet,
                            createdAt: snippet.createdAt,
                            user: snippet.user,
                            id: snippet._id
                        };
                    }),
                  foo: msg
                };
                resolve(context);
            });
        });

    }).then(function(context){
        response.render("start/index", context);
    }).catch(function(err){
        response.end(err);
    });
});

另一次尝试，这里尝试将路由器绑定(bind)到 Promise...

 router.route("/")
.get(function(request, response) {

    var session = request.session;

    return new Promise(function(resolve, reject) {

        resolve("hi!");

    }).then(function(msg){

        router.msg = msg;

        return new Promise(function(resolve, reject) {

            Snippet.find({}, function(error, data) {
                let context = {
                    snippets: data.map(function(snippet) {
                        return {
                            name: snippet.name,
                            snippet: snippet.snippet,
                            createdAt: snippet.createdAt,
                            user: snippet.user,
                            id: snippet._id
                        };
                    }),
                  foo: this.msg
                };
                resolve(context);
            }.bind(router));
        });

第三次尝试...

router.route("/")
.get(function(request, response) {

    var session = request.session;

    return new Promise(function(resolve, reject) {

        context.msg = "hi!";

        resolve(context);

    }).then(function(context){

        return new Promise(function(resolve, reject) {
            Snippet.find({}, function(error, data) {
                context.snippets = {
                    snippets: data.map(function(snippet) {
                        return {
                            name: snippet.name,
                            snippet: snippet.snippet,
                            createdAt: snippet.createdAt,
                            user: snippet.user,
                            id: snippet._id
                        };
                    }),
                };
                resolve(context);
            });
        });

    }).then(function(context){
        response.render("start/index", context);
    }).catch(function(err){
        response.end(err);
    });
});

所以基本问题始终是，当我没有周围对象，没有“this”将其附加到 =) 时，如何在 Promise 范围内“注入(inject)”或使用变量

Answer 1

利用bluebird强大的 Promise.coroutine() :

router.route("/")
.get(Promise.coroutine(function*(request, response) {
    try {
        const dbResult1 = yield getValueFromDatabaseAsync(/* w/e */);
        const dbResult2 = yield getValueFromDatabaseAsync(/* w/e, can be dbResult1 */);

        const context = {...}; // use dbResult1 and dbResult2 here normally
        response.render("start/index", context);
    } catch (err) {
        response.end(err);
    }
}));