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Maps, Sets, and Traversal2026-04-07

Map​

Regular objects force keys to be strings (or symbols). A Map lets the key be almost anything, including another object.

let userConfig = new Map();
let userObj = { name: "Zayeem" };

userConfig.set(userObj, "dark theme");
console.log(userConfig.get(userObj)); // "dark theme"

This is useful when you want to attach metadata to an object without modifying that object.

Set​

A Set stores only unique values. Adding a duplicate does nothing.

let rawData = ["tech", "coding", "tech"];
let uniqueData = [...new Set(rawData)];

console.log(uniqueData); // ["tech", "coding"]

One common production use case is removing duplicates from arrays.

WeakMap​

WeakMap is useful for temporary object-linked data. If the original key object is no longer referenced elsewhere, its WeakMap entry can be garbage-collected automatically.

Engineers use this pattern to cache temporary data while reducing memory-leak risk.

Object Traversal Methods​

You cannot run a simple loop over a plain object with array methods directly. To read data efficiently, convert the object first with Object.keys, Object.values, or Object.entries.

let userStats = { posts: 10, likes: 50 };
let numbersOnly = Object.values(userStats);

console.log(numbersOnly); // [10, 50]

Object Traversal with forEach​

You cannot use forEach directly on a plain object because it is not an array. Once you convert it with Object.entries or Object.keys, you can chain forEach immediately.

let serverConfig = { port: 8080, status: "active" };

// Convert to an array of pairs, then loop through each pair.
Object.entries(serverConfig).forEach(([key, value]) => {
  console.log(key, value);
});

Map Traversal with forEach​

Unlike plain objects, Map already has forEach. The callback receives value first, then key.

let userRoles = new Map();
userRoles.set("Zayeem", "Admin");
userRoles.set("Alex", "Editor");

// Notice the order: value comes before key.
userRoles.forEach((role, name) => {
  console.log(name, role);
});

Destructuring and The Rest Pattern​

Destructuring lets you pull out only the fields you need from large objects, rename fields, and collect the remaining fields.

let apiResponse = { id: 1, title: "Hello", author: "Zayeem", views: 500 };

// Pull title, rename views to totalViews, and collect the rest.
let { title, views: totalViews, ...otherData } = apiResponse;

console.log(title); // "Hello"
console.log(totalViews); // 500
console.log(otherData); // { id: 1, author: "Zayeem" }

Mental Model​

  • If you need guaranteed unique items, use a Set.
  • If you need keys that are not strings, use a Map.
  • If you need cache-like object keys with safer memory behavior, use a WeakMap.
  • If you are handling large API responses, use destructuring to keep variables focused and clean.