Chapter 3 · Data

Objects

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Step 1 of 22 · Concept

An object holds values by name

By the end of this lesson you keep records, loop over them, and save them as text.

const p = { name: "Ada", age: 36 };
const key = "age";
  • An object holds values by name. Each property is a key and its value.
  • p.name is "Ada". p[key] reads the key in the box key: 36.
  • A key with a space needs quotes: { "first name": "Ada" }, read as x["first name"].
  • On the heap, as an array: the box holds an arrow.
const p = { name: "Ada" };
p.age = 36;
delete p.name;
console.log(p.name);
console.log(p.city ?? "none");
console.log(p);

const prices = { tea: 2.5, cake: 3 };
console.log(Object.keys(prices).join(", "));
for (const [key, value] of Object.entries(prices)) {
  console.log(`${key}: ${value.toFixed(2)}`);
}

const s = {};
s.Zoe = 3;
s["10"] = 1;
s.Ann = 2;
console.log(Object.keys(s).join(" "));

const prices = { tea: 2.5, cake: 3, juice: 1.75 };
let total = 0;
// log each item and its price; add it to total
console.log(`Total: ${total.toFixed(2)}`);

const a = { score: 1, tags: ["x"] };
const b = a;
const c = { ...a };
b.score = 2;
c.tags.push("y");
console.log(a.score, c.score);
console.log(a.tags.join(","));

const text = `[{"name":"Ada","score":90},
  {"name":"Alan","score":85}]`;
const people = JSON.parse(text);
for (const p of people) {
  const { name, score } = p;
  console.log(`${name}: ${score}`);
}
function record(name, score) {
  return { name, score };
}
people.push(record("Grace", 95));
console.log(JSON.stringify(people[2]));

import { readFileSync } from "node:fs";
const text = readFileSync(0, "utf8");
// 1. parse the records
// 2. log each one: Alan: 85
// 3. log the best: Best: Ada

const ada = { name: "Ada", tags: ["math"] };
const copy = { ...ada };
copy.tags.push("code");
console.log(ada.tags.join(", "));
console.log(copy.tags.join(", "));
Terminal
Step 1 of 8Add, delete, and what is missing

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