ES proposal: Object.entries() and Object.values()
The following ECMAScript proposal is at stage 4: “Object.values/Object.entries” by Jordan Harband. This blog post explains it.
Object.entries()
This method has the following signature:
Object.entries(value : any) : Array<[string,any]>
If a JavaScript data structure has keys and values then an entry is a key-value pair, encoded as a 2-element Array. Object.entries(x)
coerces x
to an Object and returns the entries of its enumerable own string-keyed properties, in an Array:
> Object.entries({ one: 1, two: 2 })
[ [ 'one', 1 ], [ 'two', 2 ] ]
Properties, whose keys are symbols, are ignored:
> Object.entries({ [Symbol()]: 123, foo: 'abc' });
[ [ 'foo', 'abc' ] ]
Object.entries()
finally gives us a way to iterate over the properties of an object (read here why objects aren’t iterable by default):
let obj = { one: 1, two: 2 };
for (let [k,v] of Object.entries(obj)) {
console.log(`${JSON.stringify(k)}: ${JSON.stringify(v)}`);
}
// Output:
// "one": 1
// "two": 2
Setting up Maps via Object.entries()
Object.entries()
also lets you set up a Map via an object. This is more concise than using an Array of 2-element Arrays, but keys can only be strings.
let map = new Map(Object.entries({
one: 1,
two: 2,
}));
console.log(JSON.stringify([...map]));
// [["one",1],["two",2]]
FAQ: Object.entries()
Why is the return value of
Object.entries()
an Array and not an iterator?
The relevant precedent in this case isObject.keys()
, not, e.g.,Map.prototype.entries()
.Why does
Object.entries()
only return the enumerable own string-keyed properties?
Again, this is done to be consistent withObject.keys()
. That method also ignores properties whose keys are symbols. Eventually, there may be a methodReflect.ownEntries()
that returns all own properties.
Object.values()
Object.values()
has the following signature:
Object.values(value : any) : Array<any>
It works much like Object.entries()
, but, as its name suggests, it only returns the values of the own enumerable string-keyed properties:
> Object.values({ one: 1, two: 2 })
[ 1, 2 ]
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