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Object API: Object.fromEntries() in depth

Object.fromEntries() converts key-value pairs into an object

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Object.fromEntries() in depth

  • it is a static function defined on Object

the key-value pairs might be given as

  • a simple array, with (at least) two values
  • a simple array, with multiple key-value pairs
  • a simple array, with duplicate keys override their predecessor
  • an object, with properties 0 and 1
  • an object, without a property 0 results in an object using undefined as key
  • many objects, with the same property 0 override each other`s values
  • a Map, which maps naturally to object key+value
  • an empty Map, results in an empty object
  • a Set, used with set.entires(), returns an object with key=value

the 1st parameter

must be iterable

  • like an array
  • a self-made iterable

each entry must have a property 0 and 1 for key+value

  • like an object with these explicit properties
  • if any (or both) are missing, undefined is used
  • if an empty array is given, both are undefined too
  • toString-ables can be keys
  • iterables (like a Map) are NOT expected, and not used

throws a TypeError

  • if missing
  • if undefined (which is not an iterable)
  • if null (which is not an iterable)
  • if boolean (which is not an iterable)
  • if a Symbol (which is not an iterable)

more use cases/learnings, are

  • can be used to map data
  • an empty string, returns an empty object
  • a single-space string, throws

not symetric to Object.entries()

  • allows Symbols as keys, while Object.entries() does not report them


Description of Object.fromEntries() on MDN.
The specification describing `Object.fromEntries()`.
The (now archived) proposal, before it went into the spec, interesting read if you wanna go deep.
Very interesting details about some decisions on this method.