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Built InsObject.fromEntries()

Object.fromEntries()

Static method that converts an iterable of [key, value] pairs into a plain object. The inverse of Object.entries().

When to use it

  • FormData to object — Object.fromEntries(new FormData(form))
  • Map to object — Object.fromEntries(map)
  • Transforming objects — entries → map/filter → back to object
  • URL params to object — Object.fromEntries(new URLSearchParams(...))

Anatomy

// from an array of pairs
Object.fromEntries([['a', 1], ['b', 2]])
// { a: 1, b: 2 }
 
// from a Map
const map = new Map([['x', 10], ['y', 20]])
Object.fromEntries(map)
// { x: 10, y: 20 }
 
// from FormData
Object.fromEntries(new FormData(form))
// { name: '...', email: '...' }
 
// from URLSearchParams
Object.fromEntries(new URLSearchParams('?page=1&sort=name'))
// { page: '1', sort: 'name' }

At a glance

Input typeWhat you get
[['key', 'value'], ...]Plain object from array of pairs
MapPlain object from Map entries
FormDataPlain object from form fields
URLSearchParamsPlain object from query string

1. Simple — array of pairs to object

const pairs = [['name', 'Alice'], ['role', 'admin']]
const obj = Object.fromEntries(pairs)
// { name: 'Alice', role: 'admin' }

2. Intermediate — transform an object

const prices = { apple: 1.2, banana: 0.5, cherry: 2.0 }
 
// uppercase all keys
const upper = Object.fromEntries(
  Object.entries(prices).map(([key, val]) => [key.toUpperCase(), val])
)
// { APPLE: 1.2, BANANA: 0.5, CHERRY: 2 }
 
// filter out cheap items
const premium = Object.fromEntries(
  Object.entries(prices).filter(([, val]) => val > 1)
)
// { apple: 1.2, cherry: 2 }

3. Advanced — query string round-trip

// parse a query string into an object
const params = new URLSearchParams('?page=2&sort=name&order=asc')
const obj = Object.fromEntries(params)
// { page: '2', sort: 'name', order: 'asc' }
 
// modify and serialize back
obj.page = '3'
const qs = new URLSearchParams(obj).toString()
// 'page=3&sort=name&order=asc'

Gotchas

Duplicate keys — last one wins

Object.fromEntries([['a', 1], ['a', 2]])
// { a: 2 } — the second pair overwrites the first
 
// same issue with FormData checkboxes — use .getAll() instead

All values become the value type — no coercion

const params = new URLSearchParams('?count=5&active=true')
const obj = Object.fromEntries(params)
 
obj.count  // '5' — string, not number
obj.active // 'true' — string, not boolean
 
// parse manually if you need typed values
  • Object — Object.entries() is the inverse
  • FormData — common input for fromEntries
  • .map() — used in the entries → transform → fromEntries pattern
  • .reduce() — alternative way to build objects from arrays
Last updated on
FormData.map()