2-11 Undefined

千萬不要自己將變數的值設成undefined，以保證undefined都是js engine 所設的，方便debug

作者覺得叫notset 比較合適 😤

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console.log(a);

//Uncaught ReferenceError: a is not defined
  • a does not exist in memory
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var a;

console.log(a);

//undefined
  • a exists in memory, but is set to a special value - undefined
testing undefined value
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var a;

if (a === undefined){
  console.log("a is not defined");
}
else{
  console.log("a is definded");
}