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The typeof operator returns the type of the argument. It’s useful when we want to process values of different types differently or just want to do a quick check.
A call to typeof x returns a string with the type name:
typeof undefined
"undefined"
typeof 0
"number"
typeof 10n
"bigint"
typeof true
"boolean"
typeof "foo"
"string"
typeof Symbol("id")
"symbol"
typeof Math
"object"
typeof null
"object"
typeof alert
"function"