串联执行函数

Question

您能否向我解释一下，串行执行函数是如何工作的？
我浏览了 Connect 的源代码，但我不明白。

我想自己写一个。

app.get(
  '/',
  function(req, res, next) {
    // Set variable        
    req.var = 'Happy new year!';
    // Go to next function
    next();
  },
  function(req, res, next) {
    // Returns 'Happy new year!' 
    console.log(req.var); // <- HOW IS THIS POSSIBLE?
    // (...)
  }      
);

Answer 1

看起来好像您提供的第一个函数参数首先被 get() 函数调用。

调用时，会向调用提供 3 个参数。在调用内部，req 参数必须是可以为其分配属性的对象。您已分配 var 属性，并为其指定值 'Happy new Year!'。

您传递的下一个函数参数是通过调用 next() 参数来调用的，并且再次向该调用提供 3 个参数。第一个参数显然与分配给 var 属性的第一次调用的对象相同。

因为它（显然）是同一个对象，所以您分配的属性仍然存在。

这是一个简单的示例： http://jsfiddle.net/dWfRv/1/ (open您的控制台）

// The main get() function. It has two function parameters.
function get(fn1, fn2) {
      // create an empty object that is passed as argument to both functions.
    var obj = {};
      // create a function that is passed to the first function, 
      //   which calls the second, passing it the "obj".
    var nxt = function() {
        fn2(obj); //When the first function calls this function, it fires the second.
    };
      // Call the first function, passing the "obj" and "nxt" as arguments.
    fn1(obj, nxt);
}

// Call get(), giving it two functions as parameters
get(
  function(req, next) {
      // the first function sets the req argument (which was the "obj" that was passed).
    req.msg = 'Happy new year';
      // the second function calls the next argument (which was the "nxt" function passed).
    next();
  }, 
  function(req) {
     // The second function was called by the first via "nxt",
     //   and was given the same object as the first function as the first parameter,
     //   so it still has the "msg" that you set on it.
    console.log(req.msg);
  }
);

请注意，这是一个非常简化的示例，函数中的参数较少。也不是因为 var 是保留字，所以我将 var 更改为 msg。

Answer 2

如果需要，请尝试使用异步模块。它使能够串联、并联运行或使用池变得更加容易。

https://github.com/caolan/async