为什么我不能内联调用 res.json? [英] Why cant I inline call to res.json?

问题描述

我有 expressjs 应用程序，在特定的路由上，我调用了一个函数，该函数通过调用 res.json 以数据库文档作为参数来响应数据库中的用户.我使用基于 Promise 的库，并且我想内联回调，我将数据库文档放入响应中.但是当我这样做时程序会失败.有人可以解释为什么吗?我也想知道为什么对 console.log 的内联调用确实有效.res.json 和 console.log 这两种方法之间有什么根本区别吗?

I have and expressjs application and on a specific route I call a function that responds with a user from the database by calling res.json with the database document as parameter. I use promise based libraries and I wanted to inline the callback where I am putting the database document in the response. But the program fails when I do so. Can somebody explain why? I also wonder why inlined calls to console.log actually do work. Is there some fundamental difference between the two methods res.json and console.log?

这里有一个例子，说明什么有效，什么无效.假设 getUserFromDatabase() 返回用户文档的承诺.

Here is an example of what works and what does not work. Assume getUserFromDatabase() returns a promise of a user document.

//This works
var getUser = function(req, res) {
    getUserFromDatabase().then(function(doc) {
        res.json(doc);
    });    
} 

//This does not work (the server never responds to the request)
var getUserInline = function(req, res) {
    getUserFromDatabase().then(res.json);    
} 

//This works (the object is printed to the console)
var printUser = function(req, res) {
    getUserFromDatabase().then(console.log);    
} 

推荐答案

json 函数在这样使用时会失去其正确的 this 绑定，因为 .then 将直接调用它而不引用 res 父对象，因此绑定它:

The json function loses its correct this binding when used like that since .then is going to invoke it directly without reference to the res parent object, so bind it:

var getUserInline = function(req, res) {
    getUserFromDatabase().then(res.json.bind(res));    
}

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