The Phonebook (JS style)

readstream_no_boundary.js

const fs = require("fs");

const contactsReadStream = fs.createReadStream("./contacts.txt", {
  encoding: "utf8",
});

contactsReadStream.on("data", (data) => {
  const phonebook = {};
  let output = "";

  data.split("\n").map((line) => {
    const trimmedLine = line.trim();
    const split = trimmedLine.split(" ");
    const hasNumber = split.length === 2;

    if (hasNumber) {
      phonebook[split[0]] = split[1];
    } else {
      if (phonebook[split[0]]) {
        output = `${output}\n${split[0]}=${phonebook[split[0]]}`;
      } else {
        output = `${output}\nNot Found.`
      }
    }
  });

  console.log(output);
});

readstream_w_all_info.js

const fs = require("fs");

const contactsReadStream = fs.createReadStream('./contacts.txt', {encoding: 'utf8'})

contactsReadStream.on("data", (data) => {
  
  const phonebook = {};
  const [boundary, ...contactInfo] = data.split("\n").map((line) => line.trim());
  
  let i = 0;

  while (i < +boundary) {
    const [name, number] = contactInfo[i].split(" ");
    phonebook[name] = number;

    i++;
  }

  const lookupCollection = contactInfo.slice(+boundary);

  let output = "";

  for (let lookupName of lookupCollection) {
    if (phonebook[lookupName]) {
      output = output + `\n${lookupName}=${phonebook[lookupName]}`
    } else {
      output = output + "\nNot found";
    }
  }

  console.log(output);
})

What if they are all inter-mingled?

What if it was a really huge file where the stream could only do parts at a time?