Manually implement function isinstanceof (child, parent)

1-1 instance Main functions and use

Determine whether an instance belongs to a certain type

let person = function(){
    
}
let no = new person();
no instanceof person;    // true

1-2 insInstanceOf principle

Before understanding the principle and hand code , Need to know JS Prototype chain ： JS Prototype chain

1、 The main implementation principle ： Just the variables on the right prototype On the prototype chain of the left variable

• so,instanceof In the process of searching, we will traverse the prototype chain of variables on the left , Until you find the... Of the variable on the right prototype
• To find the failure , return false, That is, the left variable is not an instance of the right variable

2、

function new_instanceOf(leftValue, rightValue) {
    
	let rightValue = rightValue.prototype;      //  Take... Of the right expression prototyoe value 
	leftValue = leftValue.__proto__;
	while (true) {
    
		if  (leftValue === rightProto) {
    
			return true;
		} 
		if (leftValue === null) {
    
   		return false;
   	}
		leftVaule = leftVaule.__proto__ ;
	}
}

1-3 Manual code and test stage

function instance_of(l,r) {
    
	let rProto = r.prototype;
	let lValue = l.__proto__;
	while(true) {
    
		if (lValue === null) {
    
			return false;
		}
		if (lValue === rProto) {
    
			return true;
		}
		lValue = lValue.__proto__;
	}
}
//  Start testing 
var a = []
var b = {
    }

function Foo(){
    }
var c = new Foo()
function child(){
    }
function father(){
    }
child.prototype = new father() 
var d = new child()

console.log(instance_of(a, Array)) // true
console.log(instance_of(b, Object)) // true
console.log(instance_of(b, Array)) // false
console.log(instance_of(a, Object)) // true
console.log(instance_of(c, Foo)) // true
console.log(instance_of(d, child)) // true
console.log(instance_of(d, father)) // true