JavaScript Variables
Relevant Knowledge Points
Variables、Scope、Memory
1. What are the differences between undeclared 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. |
2. What are global variables
JS
msg = "Hello"; // var is missing, it becomes global variable