Uncaught TypeError: Cannot assign to read only property

Question

I was trying out this really simple example from the awesome "Professional JavaScript for Web Developers" book by Nicholas Zakas but I can't figure what I am doing wrong here. Must be something really simple that I missed but I'm stuck.

Here is the code:

'use strict';  var book = {};  Object.defineProperties(book, {     originYear: {         value: 2004,         writable: false     },      _year: {         value: 2004     },      edition: {         value: 1     },      year : {         get: function() {             return this._year;         },          set: function(newValue) {             if(newValue > this.originYear) {                 this._year = newValue;                 this.edition += newValue - this.originYear;             }         }     } });  console.log(book.edition); book.year = 2006; console.log(book.edition); 

The error I am getting on the Chrome console is:

Uncaught TypeError: Cannot assign to read only property '_year' of #main.js:31 Object.defineProperties.year.setmain.js:39 (anonymous function)

Can someone please explain where I have gone wrong?

Here is the fiddle

Answer 1

When you use Object.defineProperties, by default writable is set to false, so _year and edition are actually read only properties.

Explicitly set them to writable: true:

_year: {     value: 2004,     writable: true },  edition: {     value: 1,     writable: true }, 

Check out MDN for this method.

writable
true if and only if the value associated with the property may be changed with an assignment operator.
Defaults to false.