Shortcuts for arrow syntax

Question

I am used to the following arrow syntax:

(var1, var2, var3) => {
    statement1;
    statement2;
    ...
}

However, I've also seen it simplified to just:

var1 => something

What are the rules for when the arrow statement can be simplified. Specifically:

• Not need parens before the =>?
• Not need { ... } around the function body?