&&在JavaScript表达式中返回什么

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

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;
}

Answer 3

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.

Answer 4

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 , empty strings... (note that strings that contain falsy things like "false" or "0000" are actually truthy). or even unreadable babble like function(){return null;}(); may evaluate as false.