Larrydx Larrydx - 2 months ago 10
Javascript Question

Uncaught TypeError: Cannot assign to read only property

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

Leo Leo
Answer

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.