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  • Ahmed Elgendy thinks this is interesting:

it is most common to use null in places where an object should be but one can’t be created or found, and it is most common to find undefined when you have a variable that hasn’t been initialized, or an object with a missing property, or an array with a missing value.

From

Cover of Head First JavaScript Programming

Note

Where would you get "null" and where would you get "undefined"?