Using Custom Javascript Code For Data Formatting

Hey everyone. I'm really struggling with this javascript function that I'm using to make sure inputs from the user are formatted properly so they are not able to input any random information. For example this is the script I have for a first name input from the user. I feel like the code may be written wrong but I'm receiving a error where it's not receiving the inputs correctly into the function and wanted to see if anyone has dealt with this before and could recommend me some steps for debugging this.