I understand that the following code wraps a number into an object:
var x = Object(5);
I therefore expect and understand the following:
alert(x == 5); //true
alert(x === 5); //false
However, I also understand that an object is a list of key/value pairs. So I would have expected the following to be different:
alert(JSON.stringify(5)); //5
alert(JSON.stringify(x)); //5
What does the structure of x look like? And why does it not appear to be in key/value pair format?
The Object constructor creates an object wrapper for the given value, of a Type that corresponds to the value.
So you get a Number object with the primitive value 5 when passing a number to Object
var x = Object(5);
it's exactly the same as doing
var x = new Number(5);
when calling valueOf() on both, you get the primitive value 5 back again, which is why stringifying it gives the same as stringifying the number 5, the object is converted to it's primitive value before stringifying
The specification for JSON.stringify says
Boolean, Number, and String objects are converted to the corresponding primitive values during stringification, in accord with the traditional conversion semantics.
var x = Object(5);
This boxes 5 as an object, so Stringify just unboxes the value.
If you assign a key/value to an object, Stringify will display as such :
var x = {};
x.foo = "bar";
This is javascript Duck Typing - basically if it looks like a duck, and sounds like a duck, it must be a duck however replace duck with a data type, such as int or collection... https://en.m.wikipedia.org/wiki/Duck_typing
In a javascript console, I entered the following:
> var x = Object(5);
> x
[Number: 5]
> JSON.stringify(5)
'5'
> JSON.stringify(x)
'5'
When you use Object(5), you are creating an object with key/value pairs. However, JSON.stringify() is turning that object into it's string representation -- "5". Calling JSON.stringify() on a literal value like the primitive number 5 also returns it's string representation -- "5". You're converting both the object and the primitive to strings, that's why they are equal.
JSON.stringify(), is that right?JSON.stringify()in your question is the result of how that particular API works. It explicitly understands that Number instances should be treated as numeric values in the resulting JSON.primitiveValueproperty, where the value is5, it's just like Pointy explains, some API's understand the difference between a "regular" object, and an object just holding a primitive value.