'&&' operator in Javascript vs in Java

Question

I am currently reading JavaScript from JavaScript: The Definitive Guide. On page 76, there is this statement written,

o && o.x // => 1: o is truthy, so return value of o.x

Being a Java programmer I want to ask, why is it returning 1 instead of 'true' In Java this was not the case, but anyway I know JavaScript is different but the Logical AND mechanism is the same everywhere.(In C it returns 1 as true.)

I am asking is, why does it makes sense for this behavior at all?
Is there any way, I can ensure to return only the true or false values?

Answer 1

As per the Spec for Binary Logical Operators

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

This is a feature used in javascript a lot, one common use case is to assign a default value to a variable if it is not defined. Like assume you are expecting a options object as a param but is not mandatory so the user may not pass it

function x(options){
    options = options || {};
    //now your can access optionx.a without fearing whether options is undefined
}

You can do something like !!(o && o.x) to always get a true/false

Answer 2

if we have:

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.

So if we have:

Var xyz = a && b; //where a is undefined(false) and b is 123, then it will allocate undefined.

In case of if statements, these values are specifically converted to boolean.

Refer the following link: "https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators"

Answer 3

You can use Boolean(o && o.x); to get true or false

Answer 4

It's returning the value of the right operand. If the value of o.x was true, it would return true. If the value of o.x was 'banana', it would return 'banana'.

var o = {x:true};
console.log(o && o.x);

Answer 5

Technically && operator implemented to return the first operand if it's value is not zero, and returns the second operand otherwise in a lot of languages including Java, C/C++, Javascript, Perl... While most of the languages accepts any type of operands, Java forces you to use boolean operands so it always returns true or false. In order to force a boolean result in Javascript use:

Boolean(a && b)