What's the difference between undefined and window.undefined in JavaScript? - Stack Overflow

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If a is undefined, this works:

if(window.a) {}

while this throws an error:

if(a)

Can someone explain why?

If a is undefined, this works:

if(window.a) {}

while this throws an error:

if(a)

Can someone explain why?

• javascript
• undefined

Share Improve this question Follow edited Apr 13, 2011 at 16:40 Pops 30.9k3737 gold badges137137 silver badges153153 bronze badges asked Mar 30, 2010 at 8:16 Andreas KöberleAndreas Köberle 111k5858 gold badges280280 silver badges307307 bronze badges 0

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1 Answer 1

Sorted by: Reset to default 10

window.a is a property of window and it's undefined. a is a variable, and it's undeclared.

To use a variable, you should first declare it using the var statement. Since you didn't declare a, the interpreter raises an error. Object properties are not needed to be explicitly declared in order to use them. Crockford writes in The Good Parts:

If you attempt to extract a value from an object, and if the object does not have a member with that name, it returns the undefined value instead.

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