callback function (callback) in JavaScript (reproduced)

The code is as follows: App.use (function (req, res, next) {var err = new Error (' not Found ');     Err.status = 404; Next (ERR); }); App is object, use is method, method parameter is aanonymous functions with parameters, the function body is given directly at the back. How does this code understand? Let's begin by understanding the concept of callback functions. First of all, in JS, the function is also an object, you can assign a value to the variable,
Can be placed as a parameter in the function's argument list. For example: The code is as follows: var dosomething = function (A, a, a, b) {return a + C;} The meaning of this code is to define an anonymous function, which, in addition to having no name, is nothing like a normal function. The anonymous function is then assigned a value to the variable dosomething. Next we call: The code is as follows: Console.log (dosomething (2,3)); This will output 5. callback function, which isIn a parameter list of another function (such as parent), passed as a parameter to the parent, and then executed at a location in the body of the parent function。         To be abstract, look at the example: the code is as follows:/To illustrate the concept of callback var doit = function (callback) {var a = 1, b = 2,     c = 3; var t = callback (A,B,C);
return T + 10; }; var d = doit (function (x, y, z) {return (x+y+z);}); Console.log (d); Define the doit function first, with a parameter callback. This callback is the callback function, the name can be arbitrarily taken. Look at the function body, first define three variables A,b,c. Then call the callback function. Finally, a value is returned. The doit function is called below. Note that the definition of doit just now, callback is not defined, so just did not know what callback is for. This is actually very well understood, when we usually define the function,
The parameter also gives a name, such as a, to use a in the body of the function, but the whole process does not know what a is, only when the function is called to specify the specific value of a, such as 2. Turn back, when calling doit,
We need to specify what the callback is. As you can see, this function completes a sum function. The execution of the above code is: Call the doit function, the parameter is an anonymous function, enter the doit function body, first define A,B,C, then execute the anonymous function just now, the argument is A,b,c, and return a T, and finally return a t+10 to D. Back to the original example, App.use (...) is a function call. We can imagine that a use method must have been defined before, but not given here. A comparison of these two examples can be understood immediately. In the use of Nodejs, Express, it is not possible for each method or function we have to find its function definition to look at. So just know what parameters are passed to the callback in that definition. Then when the method or function is called,
In the parameters we define our own anonymous function to accomplish certain functions.

callback function (callback) in JavaScript (reproduced)