Send alert if certain text is inserted in a text field

Hi everyone,

I am a total noob on javascript and with your permission I'd ask for a solution on this.

I need help on creating validation script in this scenario:

If the content inserted in a text field is "apple", then send an alert "it's red", if the content inserted is "banana", then send "it's yellow". If anything else is inserted then send an alert "I have no idea"

How should i write the validation script?

Thanks a million!