Google Script: YouTube Channel Username to Channel ID

gistfile1.js

function ytChannelId(channelName) {
  if(channelName) {
    var name = getChannelFromUrl(channelName);
    var url = "https://gdata.youtube.com/feeds/api/users/" + name + "?fields=id&alt=json";
    var result = UrlFetchApp.fetch(url);
    var data = Utilities.jsonParse(result.getContentText())

    if(typeof data['entry'] !== 'undefined' && data['entry']['id']['$t']  !== 'undefined') {
      var id = "UC" + data['entry']['id']['$t'].split('/').pop();
        
      return id;
    }
  }
  
  return '';
}

function getChannelFromUrl(url) {
  var pattern = new RegExp('^(https?:\/\/)?(www\.)?youtube\.com/(user/)?([a-z\-_0-9]+)/?([\?#]?.*)', 'i');
  var matches = url.match(pattern);
  if(matches) {
    return matches[4];
  }

  return url;
}

Good Job @daniel-barrows