Why isn't everything assignable in JavaScript?

Question

I was raised with the "everything in JavaScript object-oriented and assignable" paradigm. So I lived my life happy, until...

    var x = {};

    x.field = true;
    x.field.netType = "System.Boolean";
    
    alert(x.field.netType);

It compiles, but the alert keeps giving me 'undefined'. Why!?

Answer 1

Primitives (strings, numbers, true and false) in JavaScript are not objects. However, when they are used with . or [] as if they were objects, the language obliges by implicitly constructing object wrappers for them.

In your example, that's what happened. The assignment to the object property did actually work, so there was no error, but that wrapper object was then immediately thrown away.

On the other hand:

var x = {};

x.field = new Boolean(true);
x.field.netType = "System.Boolean";

alert(x.field.netType);

(I wouldn't advise actually doing that; using objects made from the primitive wrapper types tends to have weird effects as those values propagate into code that doesn't expect them.)

Answer 2

x.field = true; 
x.field.netType = "System.Boolean"; 

is actually working.
x.field which is a primitive boolean value is getting converted to object internally but we dont have a reference of it so immediately it becomes garbage. If we store the refrence of x.field so that it doesnt be garbage we can get the value. like this....

x.field = true;  
var y = x.field.netType = "System.Boolean"; 
alert(y); 

If you write you code like this

var x = {};

x.field = {};
x.field.netType = "System.Boolean";

alert(x.field.netType);

Then it will work.

In your code this line x.field.netType = "System.Boolean"; will throw error in `strict mode

`//Cannot assign to read only property 'netType' of true`

Why this line x.field.netType gives undefined

Objects of this type are merely wrappers, their value is the primitive they wrap and they will generally coerce down to this value as required.

JavaScript will readily coerce between primitives and objects.

var a = 'Intekhab';
 a.length;//In this case the string value is coerced to a string object in order to access the property length.

var Twelve = new Number(12); 
var fifteen = Twelve + 3; // In this case Object Twelve is coerced to a primitive value.
fifteen; //15

If JavaScript detects an attempt to assign a property to a primitive it will indeed coerce the primitive to an object. This new object has no references and will immediately become fodder for garbage collection.

var primitive = "september";
primitive.vowels = 3;
//new object created to set property 
(new String("september")).vowels = 3;



primitive.vowels;
//another new object created to retrieve property 
(new String("september")).vowels; //undefined

Answer 3

x.field is a boolean value; boolean values are primitives and are read-only. When you attempt to assign a value to x.field.netType, you are attempting to modify the value of x.field. Pointy's answer mostly explains this.

When in 'normal' JavaScript mode, this results in the value simply being undefined.

The reason why someone in the comments suggested strict mode (which you should definitely be using) is that strict mode will throw an error informing you that you are attempting to assign a value to a read-only value and prevent you from doing that instead of silently returning undefined whenever you access that property.

'use strict';
var foo = true;
foo.bar = 'qux'; // this line will throw an Error

As per Pointy's example, it does actually error, at the very least on Node 4.0. I have no idea what JS engine Pointy is using but it is not operating correctly.

> (function() { 'use strict'; (true).x = 0; })()
TypeError: Cannot assign to read only property 'x' of true
    at repl:1:38
    at repl:1:45
    at REPLServer.defaultEval (repl.js:154:27)
    at bound (domain.js:254:14)
    at REPLServer.runBound [as eval] (domain.js:267:12)
    at REPLServer.<anonymous> (repl.js:308:12)
    at emitOne (events.js:77:13)
    at REPLServer.emit (events.js:169:7)
    at REPLServer.Interface._onLine (readline.js:209:10)
    at REPLServer.Interface._line (readline.js:548:8)