what does && return in javascript expression

Ok, let's assume that this.grid.getStore().sum('fees') returns something, let's say "okay!".

now the return statement in your code is a convoluted way of saying :

if(this.grid)//this.grid is defined and doesn't evaluate as 'false'
    return this.grid.getStore().sum('fees');
else
    return this.grid;

if this hasn't got a grid, we return undefined, else we call gridStore... and return its own return. It is a common way of avoiding "undefined has no method 'gridStore'"

the VERY important part is that, in a && f(), f() will NOT be called if 'a' evaluates to false. there are many things that evaluate to false, such as any undefined variable, null, strings that contain falsy things like "false" or "0000" or even unreadable babble like function(){return null;}();

You misunderstand what && does. Let a and b be "entities". Then a && b does:

1. evaluate a
2. if a is falsy return a
3. if a is truthy evaluate b
4. return b

Example:

var f = function() {
    console.log("test");
    return 'foo';
}

> 0 && f()
0
> 1 && f()
test
"foo"

Note that in first case we didn't get console.log because f() was not evaluated because 0 is falsy. This property is important and actually

a && b != b && a

even though mathematically it should be the same (but it is not due to side-effects of evaluation).

Falsy values include: 0, false, "" (empty string), null, undefined,NaN (not a number type). I don't think there are any other possible values (someone correct me if I'm wrong). Every other object is truthy.

So in your case the code can be rewritten as:

if (this.grid) {
    return this.grid.getStore().sum('fees');
} else {
    return this.grid;
}

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.