javascript - Getting unexpected behaviour with firefox web console, equality for undefined not working fine -

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when trying write following javascript code snippet in mozilla web console getting following unexpected behaviour.please refer below image.when declared variable x undefined check evaluated true.but when defined "var a" seemingly wrong answer.i have checked chrome working fine.can please explain obscure behaviour?

enter image description here

there's global variable named a , has value. var a; declaration doesn't create new variable. try changing code to:

if (a === undefined) {     console.log("undefined true"); } else {     console.log("undefined false, = " + a); }

so can see value of variable.


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