Use conditions in javascript [duplicate]

This question already has an answer here:

• Why we use if, else if instead of multiple if block if the body is a return statement 12 answers

Can i use multiple if and then after an else like this ?

if(sky == 'blue') { ... }
if(sky == 'red')  { ... }
else { alert('Sky is not blue or red'); }

When do I need else if ?

Thanks.