Stringifier.js

function stringifier (input) {

  if (typeof input === 'string') { //test for string, empty string
    return '"' + input + '"';
  }

  if (typeof input === 'function' || typeof input === 'undefined') { // test for function or undefined
    return undefined;
  }

  if (input === null) { // test for null
    return 'null';
  }

  if (Array.isArray(input)) { // test for arrays and nested arrays
    
    let stringifiedArray = [];
    for (let element = 0; element < input.length; element++) if (input.hasOwnProperty(element)) {
      if (typeof input[element] === 'function' || input[element] === undefined) {
        stringifiedArray.push('null');
      }
      else {
        stringifiedArray.push(stringifier(input[element]));
      }
    }

    let output = '[' + stringifiedArray.join(',') + ']';
    return output;
  }

  if (typeof input === 'object') { // test for objects and nested objects

    let stringifiedObjectArray = [];
    for (let value in input) if (input.hasOwnProperty(value)) {
      if (input[value] !== undefined && typeof input[value] !== 'function') {
        stringifiedObjectArray.push(stringifier(value) + ':' + stringifier(input[value]));
      }
    }
    let output = '{' + stringifiedObjectArray.join(',') + '}';
    return output;
  }

  return input + ''; //test for numbers
}