How to convert JS object to JSON encoded string?

Question

I have tried this but it is not working properly. It is just giving regular JS string and not in the JSON format

function convert(obj) {
  let ret = "{";

  for (let k in obj) {
    let v = obj[k];

    if (typeof v === "function") {
      v = v.toString();
    } else if (v instanceof Array) {
      v = JSON.stringify(v);
    } else if (typeof v === "object") {
      v = convert(v);
    } else {
      v = `"${v}"`;
    }

    ret += `\n  ${k}: ${v},`;
  }

  ret += "\n}";

  return ret;
}

Input:

const input = {
 rules3: {
    fn1: ()=> {
     setTimeout(function abc() {console.log("aaaaaaaa")}, 3000);
    }
  }
}

Expected output:

I need to JSON.parse the converted String back. So below is an example of expected output

'const input = {
 "rules3": {
    "fn1": ()=> {
     "setTimeout(function abc() {console.log("aaaaaaaa")}, 3000)"
    }
  }
}'

This is example object: const input = { rules3: { fn1: ()=> { setTimeout(function abc() {console.log("aaaaaaaa")}, 3000); } } } — Aditya B Vijay
– Aditya B Vijay, Commented Jan 13, 2023 at 7:44
Is there a reason why you can't use JSON.stringify() on your object. You don't typically need to stringify functions, just the object data is enough — Nick Parsons
– Nick Parsons, Commented Jan 13, 2023 at 7:44

Peter Thoeny · Accepted Answer · 2023-01-13 08:25:23Z

1

As you know, JSON does not support functions and regex. It looks like you want to stringify the function, and store as JSON. Instead of handcrafting a stringify function you can use the JSON.stringify() function with a replacer helper function:

const input = {
  text: 'hi',
  numbner: 123,
  array: [1, 2, 3],
  regex: /^-?\d+(:?\.\d+)?$/,
  rules3: {
    fn1: () => {
      setTimeout(function abc() {
        console.log("aaaaaaaa")
      }, 3000);
    }
  }
}

function replacer(key, val) {
  if (typeof val === 'function' || val && val.constructor === RegExp) {
    return val.toString();
  }
  return val;
}

console.log(JSON.stringify(input, replacer, 2))

Output:

{
  "text": "hi",
  "numbner": 123,
  "array": [
    1,
    2,
    3
  ],
  "regex": "/^-?\\d+(:?\\.\\d+)?$/",
  "rules3": {
    "fn1": "() => {\n      setTimeout(function abc() {\n        console.log(\"aaaaaaaa\")\n      }, 3000);\n    }"
  }
}

In the same way you can use a replacer function to restore the object using JSON.parse().

Note that because the function and regex are turned into a string it is not deterministic what is a string, what is a function, and what is a regex. Hence you might want to annotate the stringified function and regex to make it deterministic.

Docs:

JSON.stringify(): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
JSON.parse(): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse

edited Jan 13, 2023 at 8:25

answered Jan 13, 2023 at 8:19

Peter Thoeny

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2 Comments

Aditya B Vijay Over a year ago

@PeterTheony I am having issues with writing replacer function on parse. Could you please post the code?

Peter Thoeny Over a year ago

@AdityaBVijay: Can you show in your question what you have tried? SO is not a free service to produce code

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