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Undeclared variables are those that have not been declared or defined in the current scope, while undefined variables are those that have been declared but not given a value. Understanding the difference between these two terms is crucial for writing correct and effective JavaScript code.
Below are the major differences between undeclared(not defined) and undefined variables,
undeclared
These variables do not exist in a program and are not declared
If you try to read the value of an undeclared variable, then a runtime error is encountered
undefined
These variables declared in the program but have not assigned any value
If you try to read the value of an undefined variable, an undefined value is returned.