Objects and JSON

Objects and arrays

Objects group named values; arrays represent ordered collections:

const student = {
  name: 'Ada',
  scores: [98, 100],
  summary() {
    return `${this.name}: ${this.scores.length} scores`;
  }
};

student.name;
student['scores'];
student.summary();

Use destructuring for the values you need:

const { name, scores } = student;
const [firstScore, secondScore] = scores;

Optional chaining avoids an error when an intermediate value is missing:

const city = student.address?.city ?? 'Unknown city';

Objects and arrays are references. Mutating a reference changes the same object; spreading creates a shallow copy:

const updated = { ...student, name: 'Grace' };
const copiedScores = [...student.scores];

JSON is text

JSON is a language-independent text format commonly used in HTTP bodies. A JavaScript object and its JSON representation are different values:

const data = { name: 'Ada', active: true };
const text = JSON.stringify(data);
const restored = JSON.parse(text);

JSON supports strings, finite numbers, booleans, null, arrays, and objects. Functions and undefined are not represented as JSON values; dates and other special objects need an explicit representation such as a string.

Continue with Async HTTP for requests and DOM and events for rendering data.