Javascript Undefined, which you may not know

Java code
Var abc;
 
Alert (abc); // output undefined directly to indicate that the variable is not initialized.
Alert (abc = undefined) // Output true here
 
Var jeo = undefined;
Alert (jeo); // undefined is also output here, which indicates that if undefined is initialized to the variable, the undefined effect is the same as the undefined effect.
Alert (jeo = undefined); // Output true here to prove that the uninitialized value of the variable is undefined again
Alert (typeof abc); // here the output is also undefined
 
Var undefined = "What is output here? ";
Alert (undefined); // output here: "What is output here? "It should be easy to understand here
Alert (undefined = undefined); // but what is output below, I guess many of you have not tried it. Is it surprising that true is output here? Here is what we should pay attention to most.
 
// Take a look at the following
Alert (job); // print this directly. I guess everyone knows that the 'job undefined 'error will be reported.
Alert (typeof job); // the output of undefined is shown here. Through the comparison above, we can easily conclude that the operation of unspecified variables except typeof is not allowed.

Author: "LoveBeyond"