Object API: Object.fromEntries() in depth

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

Links

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