Object.entries, fromEntries & immutability

Reshape objects

Lesson 6 of 20 · 10 min · Module 2: Work with collections

API responses and settings often need small reshaping: keep a few fields, convert every value, or rename keys. Converting an object to entries makes array tools available for objects.

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By the end, you’ll be able to…

Turn objects into entries and back
Transform keys and values
Return new objects instead of changing inputs

The idea

Object.entries(obj) returns [key, value] pairs. After map or filter, Object.fromEntries(pairs) builds a new object. The original object stays unchanged.

const prices = { tea: 2, cake: 4 };
const doubled = Object.fromEntries(Object.entries(prices).map(([k, v]) => [k, v * 2]));
console.log(doubled);

01Entries are arrays

Object.entries({a: 1}) is [["a", 1]]. Once you have an array, map, filter, sort and reduce all apply, and destructuring ([key, value]) keeps the code readable.

02Allow-lists are safer

pick(user, ["id", "name"]) copies only fields you chose. Sending a whole user object to a page or an API risks leaking a field such as password.

const user = { id: 1, name: "Ana", password: "x" };
const { password, ...safe } = user;
console.log(safe);

Output

{"id":1,"name":"Ana"}

03Own properties only

Every object inherits keys like toString. Object.hasOwn(obj, key) is true only for keys stored on the object itself, so pick never copies inherited functions.

Your turn

Write mapValues(obj, fn) that returns a new object with fn(value, key) for every value. Write pick(obj, keys) that returns a new object with only the listed keys that exist in obj. Neither function may change its input.

What the checks look for

mapValues transforms every value and passes the key
mapValues does not change the input
pick keeps only existing listed keys
pick keeps falsy values and ignores inherited keys

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Take this with you

Entries turn object reshaping into array work.

Go a little further

Write omit(obj, keys), the opposite of pick, and use it to remove secrets before logging an object.