What is the best practice for testing code to pinpoint a logic error within a script?

Juan is testing his JavaScript application, which includes the following code:

Assuming Juan enters August for his birth month and his name for first name, what is the value of birthMonth after executing this code?

Loni needs to create a scriptable button in her Web form that does not submit the form but instead, calls a custom function What HTML code should she use?
A)

B)

C)

D)

Consider the following code:

Charies wants to write code to execute the changeOption function after the user chooses an option in the select menu. What change to the code should he make?

Consider the following code:

What will be displayed for the computer inventory after the Update Inventory button is clicked twice?