Converting object and/or nested array with objects to valid JSON string representation without using JSON.stringify?

Question

I'm having trouble outputting a valid JSON string from an object as input without using JSON.stringify().

Here is my current complete implementation -

var my_json_encode = function(input) {

  if(typeof(input) === "string"){
      return '"'+input+'"'
  }
  if(typeof(input) === "number"){
      return `${input}`
  }

  if(Array.isArray(input)) {
     const formatedArrayMembers = input.map(value => my_json_encode(value)).join(',');
     return `[${formatedArrayMembers}]`;
  }

  *****Trouble is here*******
  if(typeof(input) === "object" && !Array.isArray(input)) {
    let temp = "";
    for (let [key, value] of Object.entries(input)) {
            let val = `${key} : ${value}`;
            temp +=  my_json_encode(val)
    }
    return `{${temp}}`
  }
}

Current input is -> {"key1":"val1","key2":"val2"}
Expected output is -> {"key1":"val1","key2":"val2"}

Current output using object type check in my_json_encode -> {"key1 : val1""key2 : val2"}

I feel that I'm close but something is missing in my logic, I've started at this for to long and need some guidance.

If I can get my object encoder to work, I'm sure I can recursively use it to check more complicated inputs such as:

Expected Output-> [1,"a",{"key1":"value1","key2":null,"key3":[4,"b"],"key5":{"inner1":"innerval1","inner2":9}}]

Related question I asked for an array to JSON string was solved here

If you're only missing the comma between key/value pairs then why not simply change temp to an array, push your pairs to it as a string and then return with temp.join(',') ? — Tim Sheehan
– Tim Sheehan, Commented Apr 28, 2020 at 5:11
If this is just a learning exercise, this is great. But note that JSON.stringify handles quite a few edge cases not included here, things like rejecting circular data structures and not including undefined properties, functions, or Symbols, plus additional features such as a replacer function, pretty-printing, converting Infinity and NaN to null, and calling objects' .toJSON methods. A full replacement for all this is a non-trivial task. — Scott Sauyet
– Scott Sauyet, Commented Apr 28, 2020 at 13:09
Yes its a learning exercise, I'm in the process of interview prepping and this was a question that might be asked by companies. — sisternight438
– sisternight438, Commented Apr 29, 2020 at 0:14

CertainPerformance · Accepted Answer · 2020-04-28 07:35:13Z

The main issue is that you need to put "s around the whole key when iterating over the entries, and call my_json_encode on the value:

"${key}: ${my_json_encode(value)}"

You also need each key-value pair like above to be joined by ,, which can be done easily by mapping each key-value pair to the above sort of string, then .join(',')ing them.

You should also be escaping any "s in the keys or string values with a backslash. Also note that typeof is an operator, not a function - you can use it like typeof someVar:

var my_json_encode = function(input) {

  if (typeof input === "string") {
    return '"' + input.replace(/"/g, '\\"') + '"'
  }
  if (typeof input === "number") {
    return input;
  }

  if (Array.isArray(input)) {
    const formatedArrayMembers = input.map(my_json_encode).join(',');
    return `[${formatedArrayMembers}]`;
  }
  if (input === null) return 'null';
  // then it's a non-array object
  const keyValStrArray = Object.entries(input).map(([key, val]) => (
    `"${key.replace(/"/g, '\\"')}":${my_json_encode(val)}`
  ));
  return `{${keyValStrArray.join(',')}}`
};

console.log(my_json_encode({ "key1": "val1", "key2": "val2" }));
console.log(my_json_encode([1,"a",{"key1":"value1","key2":null,"key3":[4,"b"],"key5":{"inner1":"innerval1","inner2":9}}]));

Nick · Accepted Answer · 2020-04-28 05:17:14Z

For objects, you can make temp an array and just push key: my_json_encode(value) pairs to it, then joining them with , and outputting { and } around the result:

var my_json_encode = function(input) {

  if (input === null) {
    return "null";
  }

  if (typeof(input) === "string") {
    return `"${input}"`;
  }
  if (typeof(input) === "number") {
    return `${input}`;
  }

  if (Array.isArray(input)) {
    const formatedArrayMembers = input.map(value => my_json_encode(value)).join(',');
    return `[${formatedArrayMembers}]`;
  }

  if (typeof(input) === "object") {
    let temp = [];
    for (let [key, value] of Object.entries(input)) {
      temp.push(`"${key}" : ${my_json_encode(value)}`);
    }
    return `{${temp.join(', ')}}`;
  }
}

console.log(my_json_encode({key1:"val1",key2:3}));
console.log(my_json_encode([1,"a",{"key1":"value1","key2":null,"key3":[4,"b"],"key5":{"inner1":"innerval1","inner2":9}}]));

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