A simple Object-to-querystring serializer

querystring.js

function isObject(val) {
  // adapted from https://github.com/jonschlinkert/isobject and https://github.com/jonschlinkert/is-plain-object
  return(val != null && typeof val === 'object' && Array.isArray(val) === false) === true &&
    Object.prototype.toString.call(val) === '[object Object]';
};

function toQueryString(obj, format) {
  return Object.keys(obj)
    .filter(key => (obj[key] != null) || (obj[key] != undefined))
    .map(key => {
      if(Array.isArray(obj[key])) { // for arrays
        if(format) { // with format
          return obj[key]
            .filter(val => (val != null) || (val != undefined))
            .map(val => {
              return `${format.replace(/{(\d+)}/, encodeURIComponent(key))}=${encodeURIComponent(val)}`
            })
            .join('&');
        } else {
          return obj[key]
            .filter(val => (val != null) || (val != undefined))
            .map(val => `${encodeURIComponent(key)}=${encodeURIComponent(val)}`)
            .join('&');
        }
      } else if(!isObject(obj[key])) { // for plain values
        if(format) {
          // if format is specified than format the object key
          return `${format.replace(/{(\d+)}/, encodeURIComponent(key))}=${encodeURIComponent(obj[key])}`
        } else {
          // else just return plain string
          return `${encodeURIComponent(key)}=${encodeURIComponent(obj[key])}`
        }
      } else { // for nested objects, recurse into the object and serialize it
        if(format) {
          // if format is specified than create a new format based on previous'
          return toQueryString(obj[key], `${format.replace(/{(\d+)}/, key)}[{0}]`)
        } else {
          return toQueryString(obj[key], `${key}[{0}]`)
        }
      }
    })
    .join('&');
}

// Some examples

// basic
toQueryString({a: 1, b: 'string', bool: true, not: false})
// logs a=1&b=string&bool=true&not=false

// nested, complex object
toQueryString({a: { b: 1, c: { e: 1, f: 2 }, d: [1,2,3] }});
// logs a[b]=1&a[c][e]=1&a[c][f]=2&a[d]=1&a[d]=2&a[d]=3