why can't I change object property in JavaScript

Question

I made a mistake with the age property on these objects and when I try to reassign different values later, it won't change it at all.

I want this to work with the Object.create Method. What should I do to fix this?

 var personProto = { calculateAge: function() { console.log(2019 - this.yearOfBirth) }, fullName: function() { console.log(this.name + ' ' + this.lastName) } } var sam = Object.create(personProto, { name: { value: "samuel" }, yearOfBirth: { value: 1092 }, lastName: { value: "max" }, job: { value: "developer" } }); sam.yearOfBirth = 1992; sam.calculateAge(); // 927

console.log(sam.calculateAge()); is giving me 927 assuming yearsOfBirth is still 1092 even if I changed it to 1992 and the output was supposed to be 27.

Answer 1

By default, properties assigned using Object.create are not writable. Attempting to write to a non-writable property is a silent error in JavaScript - one of many reasons to use Strict Mode.

Here is your code again, but in Strict Mode:

 'use strict'; var personProto = { calculateAge:function(){ console.log(2019 -this.yearOfBirth) }, fullName:function(){ console.log(this.name + ' ' + this.lastName) } } var sam = Object.create(personProto, { name:{value:'samuel'}, yearOfBirth:{value:1092}, lastName:{value:'max'}, job:{value:'developer'} }); sam.yearOfBirth = 1992; console.log(sam.calculateAge()); 927

Notice that it now fires an error and tells you exactly what's wrong.

To fix this, just make the properties writable.

 'use strict'; var personProto = { calculateAge:function(){ console.log(2019 -this.yearOfBirth) }, fullName:function(){ console.log(this.name + ' ' + this.lastName) } } var sam = Object.create(personProto, { name:{value:'samuel',writable:true}, yearOfBirth:{value:1092,writable:true}, lastName:{value:'max',writable:true}, job:{value:'developer',writable:true} }); sam.yearOfBirth = 1992; console.log(sam.calculateAge()); 927

Answer 2

The second argument to Object.create uses property descriptors , and the writable attribute defaults to false when you don't specify it.

I would recommend to drop the second argument entirely, and instead use Object.assign to create the properties on the new object:

var sam = Object.assign(Object.create(personProto), {
    name: 'samuel',
    yearOfBirth: 1092,
    lastName: 'max',
    job: 'developer',
});

Answer 3

good luck i wish could help

 Object.assign(Object.create(personProto), {
name: 'samuel',
yearOfBirth: 1092,
lastName: 'max',