Key-Value Storage: Objects

In JavaScript, almost everything is an Object (or behaves like one). An object is a standalone entity with properties and type. A property is an association between a name (or key) and a value.

1. Object Creation and Literals

The most common way to create an object is using the Object Literal syntax.

const user = {
    id: 1,
    username: "jdoe",
    greet: function() { console.log("Hi!"); }
};

Keys: Always converted to strings or symbols.
Values: Can be any data type, including other objects or functions.

2. Accessing Properties: Dot vs. Bracket

Dot Notation (obj.prop): Clean and fast, but requires the property name to be a valid identifier.
Bracket Notation (obj["prop"]): Flexible. Allows access via variables, strings with spaces, or special characters.

3. Reference and Identity

As discussed in Module 2, objects are Reference Types. Two objects are only equal if they point to the same memory location, even if their values are identical.

const obj1 = { val: 1 };
const obj2 = { val: 1 };
console.log(obj1 === obj2); // false

const obj3 = obj1;
console.log(obj1 === obj3); // true

4. Useful Object Methods (ES6+)

Object.keys(obj): Returns an array of the object's keys.
Object.values(obj): Returns an array of the object's values.
Object.assign(target, ...sources): Copies properties from source objects to a target.
Object.freeze(obj): Makes an object immutable (shallowly).

Interview Pro-Tips: Shorthand Properties

Modern JavaScript allows you to omit the value if the key and variable name are the same. This makes your code significantly cleaner.

const username = "alice";
const user = {
    username, // Identical to username: username
    id: 1
};

Technical Summary

Prototype: Every object has a hidden [[Prototype]] link to its parent (covered in Module 19).
Complexity: Accessing properties is O(1) on average due to internal hash mapping.
Mutation: Avoid direct mutations in modern frameworks (React/Vue); prefer creating new objects using the Spread Operator ({...obj}).