Edit in JSFiddle

var fn_toggleBoolean=function(q_a){
        return Boolean(Math.abs(Number(q_a)-1));
    };

/**
 * fn_toggleBoolean(true)===false
 * fn_toggleBoolean(false)===true
 * 
 * Thus:
 * fn_toggleBoolean(fn_toggleBoolean(true))===true
 */


/**
 * Another way
 */
var fn_toggleBoolean_b=function(q_a){
        return q_a==true ? false : true;
    };


/**
 * Okay okay. The second way looks much simpler
 * but the first one is the most elegant one,
 * isn't it? :D
 */