Simple summary of a prototype and constructor in JavaScript

A constructor,
The value of the constructor is a function. In JavaScript, values of types other than null and undefined, arrays, functions, and objects, all have a constructor attribute whose value is the constructor of the value, array, function, or object. Such as:

var a = 12, //  digital 
    b = 'str', //string
    c = false, //Boolean value
    d = [1, 'd', function() { return 5; }], //  An array of 
    e = { name: 'e' }, //  object 
    f = function() { return 'function'; }; //  function 

console.log('a: ', a.constructor); // Number()
console.log('b: ', b.constructor); // String()
console.log('c: ', c.constructor); // Boolean()
console.log('d: ', d.constructor); // Array()
console.log('e: ', e.constructor); // Object()
console.log('f: ', f.constructor); // Function()

The above constructors are all built-in in JavaScript, we can also customize the constructor, such as:

function A(name) {
    this.name = name;
}

var a = new A('a');

console.log(a.constructor); // A(name)

When calling the constructor, the new keyword is needed. The constructor returns an object. See the following code:

var a = 4;
var b = new Number(4);

console.log('a: ', typeof a); // a: number
console.log('b: ', typeof b); // b: object

Second, the prototype
Prototype is a property of a function. By default, the value of a function's prototype property is an empty Object with the same name as the function. Such as:

function fn() {}

console.log(fn.prototype); // fn { }

The prototype property is mainly used to implement inheritance in JavaScript, such as:

function A(name) {
    this.name = name;
}

A.prototype.show = function() {
    console.log(this.name);
};

function B(name) {
    this.name = name;
}

B.prototype = A.prototype;

var test = new B('test');

test.show(); // test

The problem here is that the constructor for test is actually A, not B:

console.log(test.constructor); // A(name)

This is because b.pdtotype = a.pdtotype changes the constructor of b.pdtotype to A, so the constructor of b.pdtotype needs to be restored:

function A(name) {
    this.name = name;
}

A.prototype.show = function() {
    console.log(this.name);
};

function B(name) {
    this.name = name;
}

B.prototype = A.prototype;
B.prototype.constructor = B;

var test = new B('test');

test.show(); // test
console.log(test.constructor); // B(name)

The reason for this is that the value of prototype is an object, and its constructor, the value of its constructor property, is its function.

console.log(A.prototype.constructor === A); // true