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javascriptprototypeprimitive

Confusion about prototype chain , primitives and objects

in firebug console :

>>> a=12 
12
>>> a.__proto__
Number {}
>>> (12).__proto__
Number {}
>>> a.constructor.prototype === (12).__proto__
true
>>> a.constructor.prototype.isPrototypeOf(a)
false

the final line causes me a great confusion as compared to the other lines. also see Constructor.prototype not in the prototype chain?

Solution

  • When you use the . operator with a primitive, the language auto-boxes it with the appropriate Object type (in this case, Number). That's because simple primitive types in JavaScript really are not Object instances.

    Thus, the actual left-hand side of

    a.__proto__

    is not the number 12 but essentially new Number(12). However, the variable "a" continues to be the simple number value 12.

    edit — Section 8.7 of the spec "explains" this with typical ECMA 262 moon language. I can't find a clear paragraph that describes the way that a primitive baseValue is treated as a Number, Boolean, or String instance, but that section directly implies it. I think that because those non-primitive synthetic values are ephemeral (they're only "real" while the . or [] expression is being evaluated) that the spec just talks about the behavior without explicitly requiring that an actual Number is constructed. I'm guessing on that however.