How can I determine if a variable is 'undefined' or 'null'?

How can I determine if a variable is 'undefined' or 'null'?

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How to use a variable for a key in a JavaScript object literal?

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While variable is not defined - wait

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JSON to string variable dump

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JQuery/Javascript: check if var exists [duplicate]

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jQuery UI dialog button text as a variable

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jQuery return ajax result into outside variable

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JavaScript - Using a variable to refer to a property name? [duplicate]

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Variables set during $.getJSON function only accessible within function

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jQuery selectors with variables

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jQuery get text as number

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how to define variable in jquery

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Change CSS variable using jQuery [duplicate]

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jQuery: Check if character is in string

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jQuery animate of custom value (not a CSS property)

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