When looking at the output in the firebug console:
>>> a=12
12
>>> a.__proto__
Number {}
>>> (12).__proto__
Number {}
>>> a.constructor.prototype === (12).__proto__
true
>>> a.constructor.prototype.isPrototypeOf(a)
false
The last line is particularly puzzling compared to the rest. For more insights, check out this discussion on Constructor.prototype not in the prototype chain?
Using the . operator on a primitive in JavaScript automatically converts it to the appropriate Object type, such as Number. This is because simple primitive types in JavaScript are not actually instances of Objects.
Therefore, when you access
a.__proto__
the value is not just the number 12, but essentially new Number(12). Despite this, the variable "a" still remains a simple number value of 12.
edit — Section 8.7 of the spec delves into this concept using technical terminology. It doesn't explicitly state how a primitive baseValue becomes a Number, Boolean, or String instance, but it does imply it. Since these synthesized values are temporary (only existing during evaluation), the specification discusses behavior without requiring explicit construction of an actual Number. This is my interpretation based on the information provided.
@Pointy did a fantastic job in breaking it down. To ensure your final statement is accurate, it should be structured as follows:
Object.getPrototypeOf(a.constructor).isPrototypeOf(new Number(a));
When it comes to JavaScript, primitives do not possess a prototype chain like objects do. The list of primitive values includes:
As a result, calling isPrototypeOf with a primitive value will always yield a result of false.
If you attempt to use a boolean, number, or string as an object in JavaScript, the language will automatically convert it into an object for you. This is why a.constructor ultimately equates to new Number(a).constructor. Essentially, you can treat a primitive value as an object.
If you find yourself frequently needing to use a variable storing a primitive value as an object, it's recommended to explicitly make it an object. For instance, defining a as new Number(12) would be preferable. The benefits include:
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