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

3 Answers

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.

1

If sometimes a link! will not work. so create a temporary object and take all values from the writable object then change the value and assign it to the writable object. it should perfectly.

var globalObject = { name:"a", age:20
}
function() { let localObject = { name:'a', age:21 } this.globalObject = localObject;
}

I tried changing year to a different term, and it worked.

public_methods : { get: function() { return this._year; }, set: function(newValue) { if(newValue > this.originYear) { this._year = newValue; this.edition += newValue - this.originYear; } }
}

2

Your Answer

Sign up or log in

Sign up using Google Sign up using Facebook Sign up using Email and Password Submit

Post as a guest

Post Your Answer Discard

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy