How to check if username is already taken every change on textfield?

[knowncolor]

You can make a custom endpoint that checks if the username is taken or not.

Something like…

  async isUsernameAvailable(ctx) {
    let users = await strapi.entityService.findMany(
      "plugin::users-permissions.user", { filters: { username: ctx.params.username }}
    );

    return users.length === 0;
  }