Collections & Object Utilities

Set

A Set stores unique values of any type no duplicates, and (like Map) it remembers insertion order.

const ids = new Set([1, 2, 2, 3, 3, 3]);
console.log(ids); // Set(3) {1, 2, 3}  duplicates are dropped automatically

ids.add(4);
ids.has(2); // true
ids.delete(1);
console.log(ids.size); // 3

// converting to/from arrays
const arr = [...ids]; // [2, 3, 4]
const unique = [...new Set([1, 1, 2, 2, 3])]; // [1, 2, 3]  a common dedupe trick

WeakSet

Like Set, but can only hold objects (not primitives), and those objects are held weakly if nothing else references an object, it can be garbage collected even while still "in" the WeakSet. Not iterable, and has no .size. Useful for tracking membership (e.g. "has this object already been processed?") without preventing garbage collection.

const processed = new WeakSet();

function process(obj) {
  if (processed.has(obj)) return;
  processed.add(obj);
  // ... do work
}

Object static methods

Object.keys() / Object.values() / Object.entries()

const user = { name: "Ana", age: 28 };

Object.keys(user); // ["name", "age"]
Object.values(user); // ["Ana", 28]
Object.entries(user); // [["name", "Ana"], ["age", 28]]

// entries pairs well with for...of destructuring
for (const [key, value] of Object.entries(user)) {
  console.log(`${key}: ${value}`);
}

Object.assign()

Copies properties from one or more source objects into a target object (mutates the target, returns it):

const defaults = { theme: "light", fontSize: 14 };
const overrides = { fontSize: 16 };

const settings = Object.assign({}, defaults, overrides);
// { theme: "light", fontSize: 16 }  later sources win
// (the spread operator, {...defaults, ...overrides}, does the same thing for object literals)

Object.freeze() / Object.isFrozen()

Prevents adding, removing, or reassigning properties (shallow nested objects are still mutable):

const config = Object.freeze({ apiUrl: "https://api.example.com" });

config.apiUrl = "changed"; // silently ignored (throws in strict mode)
console.log(config.apiUrl); // "https://api.example.com"  unchanged
console.log(Object.isFrozen(config)); // true

Object.seal() / Object.isSealed()

Prevents adding or removing properties, but existing properties can still be reassigned (unlike freeze):

const user = Object.seal({ name: "Ana" });

user.name = "Ivan"; // OK  existing properties can change
user.age = 28; // silently ignored  can't add new properties
delete user.name; // silently ignored  can't remove properties either