Objects

What is an object?

An object is a structure that allows storing data in the form of key/value pairs.
Each key is a string, and each value can be of any type (string, number, array, another object…).

Example in English:

• An object is like a descriptive sheet with fields.

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Creating an object

let person = {
  name: "Alice",
  age: 25,
  active: true
};

console.log(person.name);  // Alice
console.log(person["age"]); // 25

Dot notation vs Bracket notation

• Dot notation (person.name) → more readable, but limited to valid identifier keys.
• Bracket notation (person["full name"]) → useful if the key contains spaces or is stored in a variable.

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Modifying an object

You can add or modify a property at any time:

let car = { brand: "Tesla", model: "Model 3" };
car.color = "red";

console.log(car);

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Iterating over an object

The for...in loop allows iterating over all the keys of an object.

let user = { name: "Bob", age: 30 };

for (let key in user) {
  console.log(key, ":", user[key]);
}

Other useful methods

• Object.keys(obj) → returns all the keys as an array.
• Object.values(obj) → returns all the values.
• Object.entries(obj) → returns [key, value] pairs.

Example:

let user = { name: "Bob", age: 30 };

console.log(Object.keys(user));   // ["name", "age"]
console.log(Object.values(user)); // ["Bob", 30]
console.log(Object.entries(user)); // [["name","Bob"], ["age",30]]

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Nested objects

Objects can contain other objects (hierarchical structure).

let person = {
  name: "Alice",
  address: {
    city: "Paris",
    zipCode: 75000
  }
};

console.log(person.address.city); // Paris

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JSON

JSON (JavaScript Object Notation) is a data format based on JS object syntax.
It is widely used for data exchange between a server and an application.

• JSON.parse(string) → converts a JSON string into a JavaScript object.
• JSON.stringify(object) → converts an object into a JSON string.

let json = '{"name": "Alice", "age": 25}';
let obj = JSON.parse(json);
console.log(obj.name); // Alice

let string = JSON.stringify(obj);
console.log(string); // {"name":"Alice","age":25}

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Best practices

• Use dot notation if possible (obj.prop), otherwise use bracket notation (obj["prop"]) when keys are not valid identifiers.
• Think of Object.keys(), Object.values(), Object.entries() to easily manipulate objects.
• Use JSON (JSON.parse / JSON.stringify) to save or transfer data.
• Organize your objects with nested objects if needed (e.g., address inside person).