Q:

is this way of saving true/false options in js efficient?

In some places, you'll se options saved as numbers. For example, when setting file permissions, you pass them using a value ranging from 0 to 7 for each group of users.
Each one of the bits in the binary representation of the number represent one of three permissions: read, write and execute, so a value of 7, with a binary representation of 111 means that the file can be red, written and executed, whereas a value of 5 means that the file can only be red and executed because of the binary representation 101.
side note: I'm not an expert on the area of how operating systems manage files, my explanation may be wrong, but I'm sure that the basic idea of binary numbers representing these options is implemented there.
What I am now wondering, if if it is efficient to store true/false options in binary using javascript. To do so, I would create the javascript as follows:
function OptionsGenerator(){
    //this method takes a number, and returns an array of options.
    this.num2options = function(num){
        var str,len,opts=[];
        if(typeof num !== "number") return false;
        str = num.toString(2);
        len = str.length;
        for(var i=0;i<len;i++){
            opts[i] = (str.charAt(i)==="1") ? true : false;
        }
        return opts;
    }
    //this function gets an array of options, and returns a number.
    this.options2num = function(opts){
        var str = "",o,opt;
        for(o in options){
            opt = (options[o]) ? 1 : 0;
            str+=opt;
        }
        return parseInt(str,2);
    }
    //this function returns a specific option with an index ranging from 1 to the number of options set. takes 2 arguments: the first one is the set of options. This can either be a number or a string containing a binary representation of the number, the second parameter contains the index of the option.
    this.getOption = function(num,optnum){
        var str;
        if(typeof num === 'number')
            str = num.toString(2);
        else
            str = num;
       return (num.charAt(optnum-1)==="1");
    }
}

my question is, is this an efficient way to save options, and if it is, is there a more efficient way to convert from binary to a number, and to get a specific option, or all the options?

A:

If you are wanting to accomplish something similar in JavaScript, I would recommend actually using bitwise operators, namely & and |.
For example:
var read = 1;
var write = 2;
var execute = 4;

var rw = read | write;

var someUserPermission = 3;  // or, var someUserPermission = read & write;

// can read? 
console.log((someUserPermission & read) === read); // => true
console.log((someUserPermission & write) === write); // => true
console.log((someUserPermission & execute) === execute); // => false

