Bitwise AND operations in Javascript

Question

I make a project that reads value from a remote JSON. In this project the remote JSON can give me with one variable 16 different types of alarm. The implementation is by a bynary 16bit value expressed in int. So if there is the third alarm it should return me 8 (bynary 1000) and if the second,eigth and tenth alarm is up it return me 1284 (binary 10100000100). Well when there is no alarm it returns me 0. So, I create a function in JS (accordly to here) that passing the value returned (8/1284/0 in the example) returns me a simple true or false if there is an alarm. The function is pretty simple:

function IsOnAlarm(passedVal) {
    if (passedVal & 0) {
        return false;
    } else {
        return true;
    }
}

but it does not function :-( I create a JSFiddle that shows the problem. How can I solve it? Thanks in advance.

If you dont want to know the specific values return passedVal!=0 — Raul Guiu
– Raul Guiu, Commented Mar 13, 2014 at 8:05

freakish · Accepted Answer · 2014-03-13 08:05:48Z

2

Well as far as I understand you just need to check whether the value is 0. There's no need for any bitwise operation.

function IsOnAlarm(passedVal) {
    return passedVal != 0;
}

Side note: passedVal & 0 is always 0 no matter what passedVal is (that's why it always goes to else).

answered Mar 13, 2014 at 8:05

freakish

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Comments

sagar43 · Accepted Answer · 2014-03-13 08:13:07Z

1

You can do it like

function IsOnAlarm(passedVal) {
    if (passedVal != 0) {
        return false;
    } else {
        return true;
    }
}

Check this http://jsfiddle.net/KQH43/3/

answered Mar 13, 2014 at 8:13

sagar43

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Comments

Etheryte · Accepted Answer · 2014-03-13 08:25:45Z

0

The AND operator always return false if one of the operands is zero, see below:

0 & 0 = 0
0 & 1 = 0
1 & 0 = 0
1 & 1 = 1

The easiest way to rewrite your function is as follows:

function IsOnAlarm(passedVal) {
    return !!passedVal;
}

Explanation: the double negation operation (!!) allows you to cast the input to boolean. Any non-zero value will evaluate to true, zero evaluates to false.

edited Mar 13, 2014 at 8:25

answered Mar 13, 2014 at 8:19

Etheryte

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Comments

setec · Accepted Answer · 2014-03-13 08:50:13Z

0

Check this jsfiddle, it is using bitwise AND comparison.

I've modified your function to make it easier to understand:

function AreAlarmsOn(alarmsToCheck, alarmState) {
    return and(alarmsToCheck, alarmState)!=0;
}

It takes alarmsToCheck and checks its bits against alarmState bits, alarmsToCheck can have bits for several alarms if you need to check multiple alarms.

answered Mar 13, 2014 at 8:50

setec

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Comments

Scott Sauyet · Accepted Answer · 2014-03-13 10:43:51Z

0

Is this what you're looking for?

var getFlags = function(val) {
    var results = [];
    for (var i = 0; i < 16; i++) {
        results.push(!!((val >> i) & 1));
    }
    return results;
};

var flags = getFlags(1284);

flags[2] //=> true
flags[8] //=> true
flags[10] //=> true
// all other values ==> false / undefined

edited Mar 13, 2014 at 10:43

answered Mar 13, 2014 at 10:37

Scott Sauyet

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