why null is not converted to a boolean value in a conditional statement?

Question

Having the following test case: (find fiddle here)

var a = new Date();
var b = null;
var c = {test: "test"};

if(a)
    console.log(a); //--- prints the current date

if(b)
    console.log('null'); //--- never reached

if(c)
    console.log('test'); //--- prints 'test'

console.log(a && b); //--- prints null

Knowing that

console.log(typeof null); //--- prints "object"
console.log(typeof c); //--- prints "object"

I expect the result of

console.log(a && b); 

to be false and not null as it shown in the example.

Any hint?

Answer 1

From the MDN:

expr1 && expr2 : Returns expr1 if it can be converted to false; otherwise, returns expr2

new Date can't be converted to false (it's not falsy), so b is returned.

Answer 2

I expect the result of

console.log(a && b);

to be false and not null as it shown in the example.

In many languages with && and || operators (or AND and OR), the result is always a boolean, yes.* But JavaScript's && and || are more useful than that: Their result is their left-hand operand's value or their right-hand operand's value, not coerced to boolean.

Here's how && works:

1. Evaluate the left-hand operand.

2. If the value is falsey (coerces to false when we make it a boolean), return the value (not the coerced value)

3. If the value from #2 is truthy, evaluate and return the right-hand value (uncoerced)

The falsey values are null, undefined, 0, "", NaN, and of course, false. Everything else is truthy.

So for instance

console.log(0 && 'hi');    // 0

...shows 0 because 0 is falsey. Note it resulted in 0, not false. Similarly, the result here:

console.log(1 && 'hello'); // hello

...is 'hello' because the right-hand side isn't coerced at all by &&.

|| works similarly (I call it the curiously-powerful OR operator): It evaluates the left-hand side and, if that's truthy, takes that as its result; otherwise it evaluates the right-hand side and takes that as its result.

The only time you get true or false from && or || is if the selected operand is already a boolean.

---

* Of course, many (but not all) of those languages (Java, C#) also require that the operands to && and || already be booleans. C's an exception, it does some coercion, but still has the result of the operands as a boolean (or if you go back far enough, an int that will be 1 or 0).

Answer 3

From MDN:

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.

MDN Documentation