Javascript add each part of username to keywords array for web-socket queries?

I am trying to add the keywords of a user’s username for faster querying using websocket events.

In my userSchema I have keywords index set to true:

  keywords: {
      // for live search of username
      type: [String],
      index: true,
    },

For example if a user’s username is ‘johnsmith’, I need to save an array with the following..

user.keywords = [j, jo, joh, john, johns, johnsm, johnsmi, johnsmit, johnsmith]

I will be doing this in my userSchema.pre(‘save’).

Below is what I am going for but how would I append the username to the parts array and then set user.keywords = parts?? Am I properly indexing the user.keywords property??

Thank you!

userSchema.pre(`save`, async function (next) {
  const user = this

  if (user.isModified(`username`)) {
    let parts = []
    for (let i = 0; i < user.username.length; i++) {
      parts.push(*???*)
    }
    user.keywords = parts
  }

  next()
})