An attempt to implement what is described and illustrated based on the first 2 minutes of https://www.youtube.com/watch?v=vlYZb68kAY0&index=8&list=PLI1t_8YX-Apv-UiRlnZwqqrRT8D1RhriX&t=164s no clue if it is right

tries-contacts.ts

const splitAt = (index: number) => (x: string) => [x.slice(0, index), x.slice(index)];
const splitFirst = splitAt(1);

class ContactNode {
  contactsCount: number = 0;
  
  data: { [letter:string]:  ContactNode } = {}

  add(contact: string) {
    this.contactsCount++;
    const [firstLetter, rest] = splitFirst(contact);

    if(!firstLetter) { return }

    if(!this.data[firstLetter]) {
      this.data[firstLetter] = new ContactNode();
    }
    this.data[firstLetter].add(rest);
  }

  findCount(contact: string) {
    const [firstLetter, rest] = splitFirst(contact)
    if(!firstLetter) {
      return this.contactsCount;
    }
    if(!this.data[firstLetter]) {
      return 0;
    }
    return this.data[firstLetter].findCount(rest);
  }
}

const allContacts = new ContactNode();

allContacts.add("sally")
allContacts.add("john");
allContacts.add("jenny");
allContacts.add("jonny")
allContacts.add("jim");
console.log(allContacts.contactsCount);
console.log(allContacts.findCount("j"));
console.log(allContacts.findCount("ji"));
console.log(allContacts.findCount("jim"));
console.log(allContacts.findCount("jo"));
console.log(allContacts.findCount("je"));
console.log(allContacts.findCount("x"));
console.log(allContacts.findCount("jx"));