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

You could use the outdated Object.defineProperty() or the Object.assign().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!"};

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

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. });

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

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.

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