when I am doing the following snippet
var name = new String("NewDelhi");
var count = new Number(10);
console.log(name instanceof String); //false
console.log(count instanceof Number); //true
when I am using name as variable it showing me false while giving it some other variable it showing true
var str = new String("NewDelhi");
var count = new Number(10);
console.log(str instanceof String); //true
console.log(count instanceof Number); //true
why is this happening.
That's because name is not a variable, it's a property in the window object. When you try to create a global variable named name, that will be ignored and the existing property will be used instead.
The type of the property is a string primitive, not a string object. The type of a variable is dynamic, so it could both hold a string primitive or a String object, but the property has a specific type and can only hold a string primitive.
typeof name will return "string", not "object". As it's not an object, it's not an instance of the String class.
I just wrote and runned a little snippet
function test(){
window.name = 'Hello World';
alert(name);
}
If you try it you will see it will output "Hello World", so your problem is caused by the scope of the variable.
When you are calling console.log(name instanceof String), you are actually doing console.log(window.name instanceof String);
Moreover window.name is a primitive string and not an object, so to check if it is a string try something like this:
alert(typeof name === "string" );
And it will output 'true'!
The previous answers don't seem to emphasize that this is a special case. This behaviour is not only because the global window object already has a name param, but because window.name (=equivalent to the global name in a browser environment) is treated as a special case, ie. always casted into a primitive string.
Other properties defined on the global window object don't have this behaviour, you can try adding a property yourself:
window.xyz = "primitive";
window.xyz = new String("NewDelhi");
window.xyz;
> String {0: "N", 1: "e", 2: "w", 3: "D", 4: "e", 5: "l", 6: "h", 7: "i", length: 8, [[PrimitiveValue]]: "NewDelhi"}
The reason it's treated as such is so that the original semantics of the property can be kept: it is the name of the window, anything but a primitive string wouldn't make sense.
See this discussion for more details (which might even warrant marking this question as a duplicate).
window which are special cases, rather than window.name specifically"name" is kind of a reserved property of the Window object.
Even though you initialize it as global variable it is ignored by default as it is already the property of Window object.
var name = new String("NewDelhi");
Now name is the property of the Window object and not the instance of String class.
The property can only hold string primitive.
Thank you.
window.name, which is a primitive, sowindow.name instanceof String === false) with silly variable nameswindow.name, which can't be an object