overwrite object property Javascript

Question

I basically having hard times understanding why I cannot overwrite an object property when using inheritance from another object, it goes like this.

var Person = function(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
};

Object.defineProperties(Person.prototype, {
  sayHi : {
   value :function() {
     return "Hi there";
   },
   enumerable : true,
   writable : true
  },
 fullName : {
   get : function() {
    return this.firstName + " " + this.lastName;  
   },
   enumerable : true
  }                      
});


var createEmployee = function(firstName, lastName, ocupation) {
  var employee = new Person(firstName, lastName);
  employee.ocupation = ocupation;  

 Object.defineProperty(employee, "sayHi", {
   sayHi : {
    value : function() {
     return "Hey I am " + this.firstName;
    }
   }
 });

 return employee;
};

var janeDoe = createEmployee('Jane', 'Doe', 'Actress');

So, accoding to what I understand, I should overwrite the sayHi property on the employee object, however, I get it as undefined.

Could please some1 show me money?

here is the jsfiddle

Best Regards.

255.tar.xz · Accepted Answer · 2019-12-06 05:08:24Z

30

You could use the outdated Object.defineProperty() or the Object.assign(). However, there's a cleaner, more modern solution to this.

Simply do:

objectName = {...objectName, property: "newvalue"};
// or
objectName = {property: "newvalue", ...objectName};

the difference between the two is that the first will overwrite the property named property, while the second will create a new property called property and assign a value, however, it won't overwrite it if it already exists.

Take a look at JS spread syntax for more info.

Such could also be done for setting your prototype:

Person.prototype = {...Person.prototype, sayHi: _=>"Hi there!"};

answered Dec 6, 2019 at 5:08

255.tar.xz

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2 Comments

andresmijares Over a year ago

Thank u! But this answer came like 5 years later... where were u when i needed u? Ah?? :@

255.tar.xz Over a year ago

lol! This stuff is relatively new... So even if I had seen this sooner it'd likely not have been as easy as it is now... Anyways, I hope it helps in the future!

Quentin Engles · Accepted Answer · 2014-01-30 03:54:46Z

6

Just realized the answer I gave was bad.

You don't need the name inside the object there.

Object.defineProperty(employee, "sayHi", {
sayHi : {//<- don't need this.
    value : function() {
    return "Hey I am " + this.firstName;
  }
 }//<- or this.
});

edited Jan 30, 2014 at 3:54

answered Jan 30, 2014 at 3:48

Quentin Engles

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Comments

starcodex · Accepted Answer · 2014-01-30 03:46:25Z

0

Try this:

 Object.defineProperty(employee, "sayHi", {
   value: "Hey I am " + employee.firstName
 });

Also, Mozilla's explanation on Object.defineProperty() is quite excellent, you should check it out.

answered Jan 30, 2014 at 3:46

starcodex

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