Objects & Destructuring

# Objects

Creating

const user = {
    name: "Alice",
    age: 25,
    "favorite color": "blue",     // quoted key — needed for spaces / special chars
    isAdmin: true,
};
const empty = {};
const fromCtor = new Object();    // rarely used

Access — dot vs bracket

user.name;                  // dot — known, valid identifier key
user["favorite color"];     // bracket — keys with spaces / special chars
const key = "age";
user[key];                  // bracket — DYNAMIC keys (variable name)

Add / update / delete

user.email = "a@x.com";     // add
user.age = 26;              // update
delete user.isAdmin;        // remove (rarely needed)

Shorthand properties (ES6)

When the property name matches the variable name:

const name = "Bob";
const age  = 30;
const u = { name, age };    // same as { name: name, age: age }

Computed keys — [expr]

const field = "score";
const update = { [field]: 99, [${field}_max]: 100 };
// { score: 99, score_max: 100 }

Method shorthand

const obj = {
    greet(name) { return hi ${name}; },     // = greet: function(name) {…}
};

Destructuring — pull values into variables

const { name, age } = user;                    // basic
const { name: userName } = user;               // rename
const { country = "IN" } = user;               // default value
const { address: { city } = {} } = user;       // nested + default
// In function params
function greet({ name, age = 18 }) { … }
greet({ name: "Alice" });

Spread to copy / merge

const copy   = { ...user };                    // shallow copy
const merged = { ...defaults, ...overrides };  // later wins
const next   = { ...user, age: 27 };           // immutable update

⚠️ Spread is SHALLOW — nested objects are still shared by reference:

const a = { profile: { city: "Delhi" } };
const b = { ...a };
b.profile.city = "Mumbai";
console.log(a.profile.city);    // "Mumbai" — same nested object!
// deep copy options
const deep = structuredClone(a);            // ✅ modern
const json = JSON.parse(JSON.stringify(a)); // works but loses Date/Map/undefined

Object.keys / values / entries

Object.keys(user);       // ["name","age","favorite color","isAdmin"]
Object.values(user);     // ["Alice", 25, "blue", true]
Object.entries(user);    // [["name","Alice"], ["age",25], …]
// reverse: build an object from entries
Object.fromEntries([["a", 1], ["b", 2]]);   // { a: 1, b: 2 }
// iterate
for (const [k, v] of Object.entries(user)) console.log(k, v);

Optional chaining + nullish coalescing — recap

user?.address?.city ?? "unknown";

Freeze / seal

const cfg = Object.freeze({ host: "x.com" });   // shallow immutability
cfg.host = "y.com";    // silently ignored (TypeError in strict)

Equality is by REFERENCE

{ a: 1 } === { a: 1 }     // false — different objects
const x = { a: 1 };
const y = x;
x === y                   // true — same reference

For deep equality use a library (lodash.isEqual) or compare via JSON.stringify for simple data.