How to remove anonymity from Function()s in JavaScript

Question

I have object that is returned from the server-side JavaScript, that is sent as a String, and I attempt to restore like:

/* function that determines whether or not a String should be a function (namely, if it is the string form of a function)
 * Parameters: 
 *  • str : the string to check
 * Returns: 
 *  • true if str should be a function, or false otherwise
 * NOTE: the primary use for this function is for restoring stringified member functions of objects returned from the server
 */
function shouldBeFunction(str)
{
    str = str.toString().trim();
    // str should *not* be function iff it doesn't start with 'function'
    if (str.indexOf('function') !== 0) return false;
    // str should *not* be function iff it doesn't have a '(' and a ')'
    if ((str.indexOf('(') === -1) || (str.indexOf(')') === -1)) return false;
    // str should *not* be function iff it doesn't have a '{' and a '}'
    if ((str.indexOf('{') === -1) || (str.indexOf('}') === -1)) return false;
    return true;
}

/* reviver function for stringified objects that contain stringified methods 
 * Parameters : 
 *  • key   : the key of the object
 *  • value : the value of object[key]
 */
function objectReviver(key, value)
{ 
    var DEBUG = false;
    if ((typeof(value) === 'string') && (shouldBeFunction(value))) {
        if (DEBUG) {
            console.log('function string detected on property named : ' + key);
            console.log('function text: " ' + value + '"');
        }
        // get arguments list, if there is one to get
        var argsList = value.substring(value.indexOf('(') + 1, value.indexOf(')')).trim();
        if (DEBUG) console.log('argsList == ' + argsList);
        var functionBody = value.substring(value.indexOf('{') + 1, value.lastIndexOf('}')).trim();
        if (DEBUG) console.log('functionBody == ' + functionBody);
        if (argsList) 
            return new Function(argsList, functionBody);    
        return new Function(functionBody);
    }
    return value;
}

var myObj = JSON.parse(objStringFromServer, objectReviver);

However, when I inspect all of its methods, they appear to be anonymous functions that cause problems later on in my code! (Namely, I have a Map<Object, String> that seems to deep-compare the Object specified in get() with any keys in the map, including its toString() method.) How do I fix this?

Is it possible there's an issue with declaring and retuning in the same line? See https://stackoverflow.com/questions/19145476/javascript-define-and-return-a-variable-on-one-line for similar, but with variables rather functions. — Paul, Oct 20 '17 at 21:11

score 1 · Answer 1 · answered Oct 20 '17 at 21:48

Found the inspiration for this in another post: Is there any non-eval way to create a function with a runtime-determined name?

/* function that determines whether or not a String should be a function (namely, if it is the string form of a function)
 * Parameters: 
 *  • str : the string to check
 * Returns: 
 *  • true if str should be a function, or false otherwise
 * NOTE: the primary use for this function is for restoring stringified member functions of objects returned from the server
 */
function shouldBeFunction(str)
{
    str = str.toString().trim();
    // str should *not* be function iff it doesn't start with 'function'
    if (str.indexOf('function') !== 0) return false;
    // str should *not* be function iff it doesn't have a '(' and a ')'
    if ((str.indexOf('(') === -1) || (str.indexOf(')') === -1)) return false;
    // str should *not* be function iff it doesn't have a '{' and a '}'
    if ((str.indexOf('{') === -1) || (str.indexOf('}') === -1)) return false;
    return true;
}

/* reviver function for stringified objects that contain stringified methods 
 * Parameters : 
 *  • key   : the key of the object
 *  • value : the value of object[key]
 */
function objectReviver(key, value)
{ 
    var DEBUG = false;
    var argList = "";
    if ((typeof(value) === 'string') && (shouldBeFunction(value))) {
        if (DEBUG) {
            console.log('function string detected on property named : ' + key);
            console.log('function text: " ' + value + '"');
        }
        // get arguments list, if there is one to get
        var argsList = value.substring(value.indexOf('(') + 1, value.indexOf(')')).trim();
        if (DEBUG) console.log('argsList == ' + argsList);
        var functionBody = value.substring(value.indexOf('{') + 1, value.lastIndexOf('}')).trim();
        if (DEBUG) console.log('functionBody == ' + functionBody);
        var f = new Function(
            "return function " + key + "() {\n" +
            functionBody +
            "};"
        )();
        return f;
    }
    return value;
}

var myObj = JSON.parse('{"test":"function(){alert(1);}"}', objectReviver);
console.log(myObj.test);

How to remove anonymity from Function()s in JavaScript

1 Answers1