GET Query Passthrough Handling

passthrough.js

/**
 * Querystring Passthrough
 * 		Allows UTM values to stickaround on the site until needed.
 */
{
  const allowedPassthrough = [
    "utm_medium",
    "utm_source",
    "utm_campaign",
    "utm_content",
    "utm_term",
    /* any GET query variables, */
  ]
  const allowedDomains = [
    window.location.host,
    /* otherurl.com, */
  ]
  const allowAbsoluteURls = true
  
  const generatePassthroughQuery = function(urlQueryString) {
    const urlQueryParams = new URLSearchParams(urlQueryString)
    const matchedParamsQuery = []
    urlQueryParams.forEach(function (value, key) {
      // We're trying to passthrough this key/value pair.
      if (allowedPassthrough.indexOf(key) !== -1) {
        matchedParamsQuery.push(key + "=" + value)
      }
    })
    
    if (matchedParamsQuery.length <= 0) {
      return false
    }
    
    let matchedParamsQueryString = matchedParamsQuery.join("&")
    return matchedParamsQueryString
  }

  const hrefPassthrough = function(urlQueryString) {
    // get the passthroughQuery
    let query = generatePassthroughQuery(urlQueryString)
    if (!query) {
      // if passthroughQuery is not available, then we're done.
      return
    }

    document.querySelectorAll("a[href]").forEach(function (element) {
      // Only process allowedDomains
      let isAllowedDomain = false
      for (let i = 0; i < allowedDomains.length; i++) {
        if (element.href.indexOf(allowedDomains[i]) !== -1) {
          isAllowedDomain = true
          break
        }
      }

      if (allowAbsoluteURls && element.href.indexOf("/") === 0) {
        // We're dealing with an absolute URL, therefore always supported.
        isAllowedDomain = true
      }

      // We didn't find a match
      if (!isAllowedDomain) {
        return
      }

      // Append the query and assign to the element's href.
      element.href = element.href + (element.href.indexOf("?") === -1 ? "?" : "&") + query
    })
  }
  
  // Set up the HREF updates
  hrefPassthrough(window.location.search)
}