buildURL.js

// @flow

type Params = {
  [string]: string | number | null | Array<string | number | null>
}

function encode(val: string | number) {
  return encodeURIComponent(val + '').
    replace(/%40/gi, '@').
    replace(/%3A/gi, ':').
    replace(/%24/g, '$').
    replace(/%2C/gi, ',').
    replace(/%20/g, '+').
    replace(/%5B/gi, '[').
    replace(/%5D/gi, ']')
}

function serializeParams(params: Params) {
  return Object.keys(params)
    .map(key => {
      const value = params[key]
      if (value == null) return
      return Array.isArray(value) ? [`${key}[]`, value] : [key, [value]]
    })
    .reduce(function (parts, entry) {
      if (entry == null) return parts
      const [key, value] = entry
      return parts.concat(value.filter(v => v).map(v => {
        if (v instanceof Date) {
          v = v.toISOString()
        } else if (v instanceof Object) {
          v = JSON.stringify(v)
        }
        return v ? `${encode(key)}=${encode(v)}` : v
      }))
    }, [])
    .join("&")
}

function buildURL(url: string, params: ?Params) {
  return params ? `${url}${url.indexOf("?") === -1 ? "?" : "&"}${serializeParams(params)}` : url
}