decycle.js

// Based on https://github.com/dscape/cycle/blob/master/cycle.js

module.exports = function decycle(object) {
// Make a deep copy of an object or array, assuring that there is at most
// one instance of each object or array in the resulting structure. The
// duplicate references (which might be forming cycles) are replaced with
// an object of the form
//      {$ref: PATH}
// where the PATH is a JSONPath string that locates the first occurance.
// So,
//      var a = [];
//      a[0] = a;
//      return JSON.stringify(JSON.decycle(a));
// produces the string '[{"$ref":"$"}]'.

// JSONPath is used to locate the unique object. $ indicates the top level of
// the object or array. [NUMBER] or [STRING] indicates a child member or
// property.

    var objects = [],   // Keep a reference to each unique object or array
        paths = [];     // Keep the path to each unique object or array

    return (function derez(value, path) {

// The derez recurses through the object, producing the deep copy.

        var i,          // The loop counter
            name,       // Property name
            nu;         // The new object or array

// typeof null === 'object', so go on if this value is really an object but not
// one of the weird builtin objects.

        if (typeof value === 'object' && value !== null &&
                !(value instanceof Boolean) &&
                !(value instanceof Date)    &&
                !(value instanceof Number)  &&
                !(value instanceof RegExp)  &&
                !(value instanceof String)) {

// If the value is an object or array, look to see if we have already
// encountered it. If so, return a $ref/path object. This is a hard way,
// linear search that will get slower as the number of unique objects grows.

            for (i = 0; i < objects.length; i += 1) {
                if (objects[i] === value) {
                    return {$ref: paths[i]};
                }
            }

// Otherwise, accumulate the unique value and its path.

            objects.push(value);
            paths.push(path);

// If it is an array, replicate the array.

            if (Object.prototype.toString.apply(value) === '[object Array]') {
                nu = [];
                for (i = 0; i < value.length; i += 1) {
                    nu[i] = derez(value[i], path + '[' + i + ']');
                }
            } else {

// If it is an object, replicate the object.

                nu = {};
                for (name in value) {
                    if (Object.prototype.hasOwnProperty.call(value, name)) {
                        nu[name] = derez(value[name],
                            path + '[' + JSON.stringify(name) + ']');
                    }
                }
            }
            return nu;
        }
        return value;
    }(object, '$'));
};