CODEWITHSUNDEEP
if-statementbitmask
alexkelbo
alexkelbo

Reputation: 724

How does JavaScript turn a bitmask to a boolean expression?

I'm confused with how JavaScript turns bitmasks into boolean expressions so you can use them directly in an if-statement. Say, you have two variables, x and y (using Node.JS REPL):

> var x = 0x4;

> var y = 0x5;

Now you AND them and get

> x & y
4

Now you compare to booleans:

> (x & y) == true
false
> (x & y) == false
false

But when you put the bitmask in an if-statement, it works like a charm:

> if (x & y) { z = 3; } else { z = 7; }
3
> if (! (x & y)) { z = 3; } else { z = 7; }
7

So my question is: Although the expression x & y does not compare to a boolean value, it does in an if-statement. What are the comversion rules or what am I missing here?

Upvotes: 1

Views: 204

Answers (2)

kviiri
kviiri

Reputation: 3302

Interesting question. The if part is easier to answer: all values apart from false, null, NaN, 0, undefined and the empty string "" are considered to be truthy - (x & y) isn't any of those in this case so it's considered to be truthy.

> (x & y) == true
false

Looking up from EcmaScript specs, the "abstract comparison" or == works like this when the the two operands (referred to as x and y in the document) are of differing types, with the right-hand operand being a boolean:

If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

So while most numeric values are truthy, only the value of true when casted to a numeric type is equal to true in the == sense. Testing on my own JavaScript console, I get the result 1 from +true, and likewise, 1 == true evaluates to true.

Upvotes: 1

rdodev
rdodev

Reputation: 3202

The if statement in JavaScript is quite easily one of the most debated features on the language, mostly because it does what's called a type coercion. So, it will coerce (x & y), which is a non-zero value, to true which is why you are seeing the result. When you negate it, you also get the result you want. It first coerces ( x & y ) to true, then negates it.

Upvotes: 0

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