How to interpolate/evaluate javascript object keys

rewriteProperties.js

/**
 * Will evaluate all the keys of a given object containing a plus sign "+"
 * to the expression result.
 * 
 * example:
 * var a = 2;
 * var obj = { " 'hello' + a " : true };
 * rewriteProperties(obj); // returns { hello2: true }
 * 
 */

/*jshint evil:true */
function rewriteProperties(obj) {
    if (typeof obj !== "object")
    {
        return obj;
    }
    for (var prop in obj) {
        if (obj.hasOwnProperty(prop)) {
            obj[prop] = rewriteProperties(obj[prop]); 
            if ( prop.indexOf("+") !== -1)
            {
                // a blow straight to the face of best practices
                var key = eval (prop); 
                obj[key] = obj[prop];
                delete obj[prop];
            }
        }
    }
    return obj;
}