function - Why JavaScript declared variable is in the global object before initialization? -

i've stumbled upon javascript variable behavior not explain.

according js docs on var keyword :

the scope of variable declared var enclosing function or, variables declared outside function, global scope (which bound global object).

also known global variables become properties of global object - 'window' in browser environments , 'global' in node.js means if variable declared 'var' keyword inside function becomes local , not global object.

this example proves it:

(function(){     var t = 1;     console.log(t in window, t); // outputs: false 1 }()); 

jsfiddle link

so far good. if variable not initialized become property of window object despite fact in function scope.

(function(){     var t;     console.log(t in window, t); // outputs: true undefined }()); 

jsfiddle link

why happen ? can learn details of behavior ? regular tutorials don't seem cover this.

thanks in advance.

[edit]: pointy clear scope works expected. had wrong understanding of 'in' operator behavior. according 'in' docs coerces left hand operand number or string , looks such index or property name in right hand object. in example 1 equal to

'1' in window 

which false

and in example 2 was

'undefined' in window 

which true.

the issue test code using in operator erroneously. it's testing whether name "undefined" defined in window, not "t". why? because left-hand operand of in coerced string. because "t" undefined, it's coerced string "undefined", , indeed property name present in global context.

change test to

console.log("t" in window, t);