Check if variable is undefined

This question already has an answer here:

How to determine if variable is 'undefined' or 'null'? 24 answers

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未定义是js中的虚假...它看起来像bar.baz可能是你的罪魁祸首。

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You can use typeof to check if a variable is undefined . It always returns a string .

if (typeof foo === 'undefined') {
  console.log('foo is undefined');
}

var foo = ['one', 'two', 'three'];

if (typeof foo !== 'undefined') {
  // access elements
  console.log(foo[0] + ', ' + foo[1] + ', ' + foo[2]);
}

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