function Parent() {

this.name = 'per';

}

function Child() {

this.age = 20;

}

Child.prototype = new Parent();

var child = new Child();

console.log(child.name + " " + child.age)//per 20

 function Parent(age) {

this.name = 'person';

this.age = age;

}

function Child(age) {

Parent.call(this, age)

}

var child = new Child(20)

console.log(child.name + " " + child.age)//per 20

 function Parent(age) {

this.name = 'per';

this.age = age;

}

Parent.prototype.sayAge = function () {

return this.name + ' age is ' + this.age;

}

function Child(age) {

Parent.call(this, age)

}

Child.prototype = new Parent();

var child = new Child(21);

console.log(child.sayAge())//per age is 21