Javascript bit masks

Question

I've used bit masks in Java (not Javascript) before, but it's been a while and it's making me bug out.

This is the Typescript I'm wanting to work with. Theres 3 roles,

enum Type {
    FRUIT = 0x1,
    VEGGIE = 0x2,
    BOTH = FRUIT | VEGGIE
}

class Thing {
    role:number;

    constructor(role:Type){
        this.role = role;
    }
    getRole(){
        return this.role;
    }
    is(role:Type) {
        return !!(this.role & role);
    }
}

var whatever = new Thing(Type.FRUIT);

console.log('fruit', whatever.is(Type.FRUIT));
console.log('veggie', whatever.is(Type.VEGGIE));
console.log('both', whatever.is(Type.BOTH));

// fruit true
// veggie false
// both true

I conceptually see why "both" is coming back as "true", but my bit math isn't great.

When the role is either FRUIT or VEGGIE , the others are false. When it's set to BOTH , all should be true.

Tried a few combinations of shifts and bit operations, but I can't get that output. If I try them separate it's fine, but I'd like to use combinations of bits to build.

Answer 1

When you perform a bitwise & operation the result is that the bits turned on in both values used in the expression.

For example (taken from Pro TypeScript , p 207)

a 1011
&
b 1101
= 1001

The first column, both values are "switched on (1)", so the result is also switched on. The second column, a is switched off, so the result is 0 , the third column b is off, so again a 0 and finally the last column both are on, so a 1 .

In your case, it is even simpler because you are dealing with such small numbers...

a 01 // FRUIT
&
b 11 // ALL
= 01 // FRUIT

So the result is 01 , or 1 if you have ten fingers.

If you use a double-bang !! to convert 1 to to a boolean (I call this slacker parsing ), you'll get true because 1 is truth-y. This doesn't answer the question you were really asking, which is "does this bit flag match".

This is why this.role & role === role is the correct code, because you won't get a mistake from the "truthiness" of the value 1 .

Answer 2

 return !!(this.role & role); 

您的版本可以按isAny ，但是您希望它按isAll ：

return (this.role & role) === role;