Find the User ID from the User Information List in Sharepoint

getid.js

// http://aymkdn.github.io/SharepointPlus/symbols/%24SP%28%29.html#.getUserInfo
$SP().getUserInfo("domain\\john_doe", function(info) {
  if (typeof info === "string") {
    console.log("Error:"+info); // there was a problem so we show it
  } else
    console.log("User ID = "+info.ID)
});

// query the User Information List
$SP().list("User Information List", "http://site.collection/root/dir").get({
  "fields":"ID",
  "where":"ImnName = 'Preferred, Name'" /* replace with the Preferred Name of the user */
},
  function(d) {
    console.log(d[0].getAttribute("ID"))
  }
);

// or you can use a list where the User exists thru the Created By (Author), or the Modified By, or a simple people picker
// here an example using the Created By (Author) field
$SP().list("My List").get({fields:"Author",where:"Author = '[Me]'",rowlimit:1}, function(d) {
  console.log($SP().getLookup(d[0].getAttribute("Author")).id)
})