I feel confused about the codepan running result

[johnsoncheg]

codepan demo:

var a = 1;
console.log(this.a) // undefined

jsbin&codepen demo:

var a = 1
console.log(this.a) // 1

judge from the result above, I think something has gone wrong with codepan's code. I guess it may has something to do with the strict mode

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[johnsoncheg]

When I print the window Object, the console panel show [Object window], it seems that the code executed is isolated from the page window Object unless I manually assign window.a = 1

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