Detailed explanation of the usage of javascript callback function with function as parameter

express is the use of callback functions that take functions as parameters. If you cannot understand this concept, the code of nodejs and express will be a mess. For example:

app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

app is an object, use is a method, the parameter of the method is an anonymous function with parameters, and the function body is given directly at the end. How to understand this code? Let's first understand the concept of callback function.
First of all, you must understand that in js, functions are also objects, which can be assigned to variables and placed as parameters in the parameter list of the function. For example:

var doSomething = function(a,b)
{
 return a + b;
}

This code means to define an anonymous function. Except that this anonymous function has no name, it is no different from an ordinary function. Then assign the anonymous function to the variable doSomething. Next we call:

console.log(doSomething(2,3));

This will output 5.

The callback function is placed in the parameter list of another function (such as parent), passed to this parent as a parameter, and then executed somewhere in the parent function body. It's abstract, look at the example:

// 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);

First define the doit function, which has a parameter callback. This callback is the callback function, and the name can be arbitrary. Looking at the function body, first define three variables a, b, and c. Then call the callback function. Finally returns a value.

The doit function is called below. It should be noted that when doit was defined just now, callback was not defined, so I didn't know what callback was used for. This is actually easy to understand. When we usually define a function, the parameter is only given a name, such as a, and a is used in the function body, but the whole process does not know what a is. Only when calling that function Only then specify the specific value of a, such as 2. Looking back, when calling doit, we need to specify what callback is. As you can see, this function completes a sum function.

The execution process of the above code is:

Call the doit function, the parameter is an anonymous function; enter the function body of doit, first define a, b, c, and then execute the anonymous function just now , the parameters are a, b, c, and returns a t, and finally returns 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 it is not given here. Once you compare these two examples, you can understand it immediately.

When using nodejs and express, it is impossible for us to find the function definition of every method or function and take a look. So just know what parameters are passed to callback in that definition. Then when calling a method or function, we define anonymous functions ourselves in the parameters to complete certain functions.

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