what does && return in javascript expression

Question

I was reading the javascipt code in some application and code was this

getTotalFees:function(){
        return this.grid
        &&this.grid.getStore().sum('fees');
}

Now i am confused what it will return.

IT looks to me like

return a&&b

won't it return true or false rather than b

Answer 1

Logical AND (&&):
expr1 && expr2 Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.

Source

So, basically:
If the first parameter is falsy, it returns that parameter. Else, it literally returns the second parameter.

In your case, this means that, if this.grid exists, it returns this.grid.getStore().sum('fees');

Answer 2

This is done to protect against calling a method on undefined property, witch would cause an error. So if this.grid is undefined, then undefined is returned.

In expressions if a && b when a equals to false (or in javascript it can be an expression like in Cerburs answer), then a is returned.

Similarly with || operator, the first from the left that equals to true (in javascript not 0, not undefined, not null, not NaN, and not false of course) is returned.