javascript in operator and events

update:

for( i in window)if(i=='onhashchange')console.log(i, window[i]); //prints onchangechange undefined

on browser where onhas开发者_如何学Pythonhchange event is supported I have

'onhashchange' in window;  //returns true
window['onhashchange']; //returns false
window.onhashchange;  //returns false;

why does the former returns true and the rest returns false?

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Could it be simply that the window object has a property called onhashchange, but the value of the property is currently null and therefore considered false?

var a = { 'foo': null }
console.log('foo' in a) // true
console.log(a.foo) // null
console.log(!!a.foo) // false

…where the !! is a double negation, a trick to convert value to true/false.

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I think with help of the first operation you just check if such an event is in window object, so it returns true. Two other tell you if such a handler is implemented, and there is no implementation yet you get false. Can you check the following:

window.onhashchange = function() {}
window['onhashchange'];

yea, anyway it will return true now, because we have defined that variable... So most probably

'onhashchange' in window;

just checks if browser supports it

window['onhashchange']; //returns false
window.onhashchange;  //returns false;

just checks if handler already implemented

PS. Also you can be interesting in Javascript IN operator compatibility, here @Andy E wrote:

You should err on the side of caution when using it to check event support. All implementations except Mozilla support "eventname" in element as a test for DOM events, Firefox will result in false here unless a handler is defined.

UPDATE: to see difference between "x in window" and "window.x" (which is equal to window['x']), take a look at the following script and its output:

var foo = {};
console.info( 'bar' in foo ); // false, because no such proeprty
console.info( foo.bar ); // undefined, because no such property
console.info( foo.bar ? 'true' : 'false' ); // 'false' because no such property

foo = { bar: false };
console.info( 'bar' in foo ); // true, because there is such property
console.info( foo.bar ); // false, because this is value of bar property
console.info( foo.bar ? 'true' : 'false' ); // 'false' because foo.bar is false

foo = { bar: 1 }; 
console.info( 'bar' in foo ); // true, because there is such a property
console.info( foo.bar ); // 1, because this is value of bar proeprty
console.info( foo.bar ? 'true' : 'false' ); // 'true', because foo.bar is 1 (which is not 0 which meant to be false)

foo = { bar: 0 }; 
console.info( 'bar' in foo ); // true, because there is such a property
console.info( foo.bar ); // 0, because this is value of bar proeprty
console.info( foo.bar ? 'true' : 'false' ); // 'false', because foo.bar is 0 (which meant to be false)

UPDATE2: Detecting event support without browser sniffing - this article shows how to make event detection cross browser (because "event in window" does not work in Mozilla; this article also answers why it is so)