json-stringify-function is a related post that complements this discussion.

A valuable snippet found through that post could be beneficial to individuals coming across this response. The technique leverages the replacer parameter in JSON.stringify and the reviver parameter in JSON.parse.

To elaborate, if a value happens to be of function type, the method .toString() is invoked using the replacer. During parsing, the eval() function executes via the reviver when encountering a function represented in string format.

var JSONfn;
if (!JSONfn) {
    JSONfn = {};
}

(function () {
  JSONfn.stringify = function(obj) {
    return JSON.stringify(obj,function(key, value){
            return (typeof value === 'function' ) ? value.toString() : value;
        });
  }

  JSONfn.parse = function(str) {
    return JSON.parse(str,function(key, value){
        if(typeof value != 'string') return value;
        return ( value.substring(0,8) == 'function') ? eval('('+value+')') : value;
    });
  }
}());

This code excerpt is sourced from Vadim Kiryukhin's JSONfn.js or refer to the documentation at Home Page

Lately, I've been faced with a similar requirement. Interestingly, the desired output may resemble JSON at first glance, but it's actually just JavaScript code.

While JSON.stringify usually does the job well, it can encounter difficulties when dealing with functions.

I managed to tackle this issue by employing a few clever techniques:

1. Utilizing the replacer (the second parameter of JSON.stringify())
2. Using func.toString() to extract the JavaScript code for a function
3. Maintaining a record of which functions have been converted into strings and directly replacing them in the final result

Below is an example of how this approach plays out:

// Our original data
const source = {
    "aaa": 123,
    "bbb": function (c) {
        // Perform some task
        return c + 1;
    }
};

// Track serialized functions in a list
const functions = [];

// JSON replacer - provides placeholders for functions
const jsonReplacer = function (key, val) {
    if (typeof val === 'function') {
        functions.push(val.toString());
        
        return "{func_" + (functions.length - 1) + "}";
    }
        
    return val;
};

// Regex replacer - substitutes placeholders with functions
const funcReplacer = function (match, id) {
   return functions[id];
};

// Generate JSON with placeholders and then replace those with actual functions
const result = JSON
    .stringify(source, jsonReplacer)
    .replace(/"\{func_(\d+)\}"/g, funcReplacer);

// Display the final result
document.body.innerText = result;

body { white-space: pre-wrap; font-family: monospace; }

Note: Pay close attention to the placeholder format to ensure it's sufficiently unique. If you modify it, remember to update the regex accordingly.

While this may not strictly adhere to JSON standards, it presents an interesting workaround. One might question the practical application of such a method, but for those curious enough to experiment, consider the following unconventional approach:

x.key2 = x.key2.toString();
JSON.stringify(x)  //"{"key1":"xxx","key2":"function (){return this.key1}"}"

Automating the first line could be achieved by recursively iterating over the object. However, reversing this process poses a greater challenge - discerning whether a key contains a function code as a string. While using eval is an option, one must exercise caution and informed judgment.

Here is the solution I came up with: https://gist.github.com/Lepozepo/3275d686bc56e4fb5d11d27ef330a8ed

function convertFunctionsToString(object) {
  return JSON.stringify(object, (key, val) => {
    if (typeof val === 'function') {
      return `(${val})`; // convert function to string and wrap it in parenthesis
    }
    return val;
  });
};

function convertStringToFunctions(obj) {
  return JSON.parse(obj, (k, v) => {
    if (typeof v === 'string' && v.indexOf('function') >= 0) {
      return eval(v); // evaluate and execute the function from the string
    }
    return v;
  });
};

I have devised a unique solution to handle the conversion of functions without using eval. All you need to do is insert this code snippet before utilizing any JSON methods. The method of usage remains unchanged; however, it currently only accepts one parameter, value, for converting into a JSON string. Therefore, if you provide additional replacer and space parameters, they will be disregarded.

void function () {
    window.JSON = Object.create(JSON)

    JSON.stringify = function (obj) {
        return JSON.__proto__.stringify(obj, function (key, value) {
            if (typeof value === 'function') {
                return value.toString()
            }
            return value
        })
    }

    JSON.parse = function (obj) {
        return JSON.__proto__.parse(obj, function (key, value) {
            if (typeof value === 'string' && value.slice(0, 8) == 'function') {
                return Function('return '+ value)()
            }
            return value
        })
    }
}()


// INSERT YOUR CODE BELOW

x = {
 "key1": "xxx",
 "key2": function(){return this.key1}
}

const y = JSON.parse(JSON.stringify(x))
console.log(y.key2())

The mischievous yet successful approach is to do the following:

Function.prototype.toJSON = function() { return this.toString(); }

Your main challenge (other than altering the prototype of Function) would be deserializing without relying on eval.

Creating functions from strings can be achieved without relying on the eval() function.

var obj = {a:function(a,b){
    return a+b;
}};

var serialized = JSON.stringify(obj, function(k,v){
    //special handling for function types
    if(typeof v === "function")
        return v.toString();//converting functions to strings
    return v;
});
/*output:
"{"a":"function (a,b){\n        return a+b;\n    }"}"
*/

Now, let's delve into the process of transforming a string back into a function using this method:

var compileFunction = function(str){
    //extract parameters
    var pstart = str.indexOf('('), pend = str.indexOf(')');
    var params = str.substring(pstart+1, pend);
    params = params.trim();

    //extract function body
    var bstart = str.indexOf('{'), bend = str.lastIndexOf('}');
    var str = str.substring(bstart+1, bend);

    return Function(params, str);
}

We can then utilize JSON.parse with a reviver function as shown below:

var revivedObj = JSON.parse(serialized, function(k,v){
    // a basic check to identify function strings
    if(typeof v === "string" && v.indexOf("function") !== -1)
        return compileFunction(v);
    return v;
});

//output:

 revivedObj.a

 function anonymous(a,b
 /**/) {

    return a+b;

 }

 revivedObj.a(1,2)
3

As far as I know, there are currently no serialization frameworks that are capable of persisting functions - regardless of the programming language being used. Serialization is typically used to save and restore data, while compilation is the process used to preserve functions.

When encountering structures that are almost valid JSON but contain functions, the challenge is how to stringify these structures effectively.

I came across this issue while working on a script to adjust RequireJS configurations. Here's how I tackled it. First, I inserted some code earlier in the process to ensure that an internal placeholder ("">>>F<<<") doesn't appear as a value in the configuration, just as a precaution. The input configuration is interpreted as a JavaScript Object, which can consist of arrays, atomic values, other Objects, and functions. It could be easily converted to JSON if not for the presence of functions. This configuration is represented by the config object in the subsequent code:

// Tracking encountered functions.
var functions = [];
var placeholder = ">>>F<<<";

// This function records a function object in `functions` and returns the 
// placeholder to be inserted into the JSON structure.
function handler(key, value) {
    if (value instanceof Function) {
        functions.push(value);
        return placeholder;
    }

    return value;
}

// Stringifying with our custom handler.    
var pre = JSON.stringify(config, handler, 4);

// Replacing placeholders with recorded functions.
var post = pre.replace(new RegExp('"' + placeholder + '"', 'g'),
                       functions.shift.bind(functions));

The final result is stored in the post variable. This approach relies on the order of execution of the handler being consistent with the data order in the final JSON. After reviewing the ECMAScript 5th edition specifications, I haven't identified any ordering issues in the stringification algorithm. If future editions alter this algorithm, a solution would involve using distinct placeholders for functions and referencing them back via an associative array mapping unique placeholders to functions.