JavaScript question detail
What are the differences between undeclared and undefined variables
Below are the major differences between undeclared(not defined) and undefined variables,
| undeclared | undefined |
|---|---|
| These variables do not exist in a program and are not declared | These variables declared in the program but have not assigned any value |
| If you try to read the value of an undeclared variable, then a runtime error is encountered | If you try to read the value of an undefined variable, an undefined value is returned. |
var a;
a; // yields undefined
b; // Throws runtime error like "Uncaught ReferenceError: b is not defined"
This can be confusing, because it says not defined instead of not declared (Chrome)