How javascript detects whether a variable is defined

This article mainly explains "javascript how to detect the definition of variables", the content of the explanation is simple and clear, easy to learn and understand, the following please follow the editor's ideas slowly in depth, together to study and learn "javascript how to detect whether variables are defined" bar!

Javascript method to detect whether the variable is defined: 1, use the typeof operator to obtain the data type of the variable, syntax "typeof variable name"; 2, through the "=" operator to determine whether the data type obtained is the "undefined" type, if it is not defined, otherwise it is defined.

The operating environment of this tutorial: windows7 system, javascript1.8.5 version, Dell G3 computer.

Undefined is also a special data type with only one value, indicating that it is undefined. For undeclared variables, looking at their types using the typeof operator reveals that the undeclared variable is undefined:

So we can first use the typeof operator to get the data type of the javascript variable, and then determine whether the data type obtained is of type "undefined". If so, it is not defined, and vice versa.

If (typeof b = = "undefined") {console.log ("undefined");} else {console.log ("definition");}

Output:

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