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

javascript

293126 次浏览

小开

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;
}
}
}

小开

最佳答案

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.

小开

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;
}