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typeof returns the type string value of the variable, including "object", "number", "string", "undefined", "boolean",
1. When the variable is only declared, But does not initialize the value Or returns "undefined" when the variable is not declared
> > 'undefined' > typeof e 'undefined' >
2. All reference objects return "object"
> a = > 'object' > b = String("str"> 'object' > c = Boolean(> 'object' > > var d = [] undefined > typeof d 'object' > > var e = {} undefined > typeof e 'object' >
3. Return the corresponding type "string", "number", "boolean" according to the variable value
> var a = 98undefined> typeof a'number' > var b = 'aaa'undefined> typeof b'string' > var c = trueundefined> typeof c'boolean' >
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