Why passing function object to Object.keys() returns empty array?

Question

So, functions in js are objects with some keys, right? I'm trying to iterate through them, but i'm getting empty list:

function f(a) {
console.log(Object.keys(f)) //prints []
console.log(f.arguments) //key "arguments" exists and prints Arguments object
}

Any expanation why Object.keys() doesn't return keys for function object?

Btw, f.arguments should not exist, it's deprecated and accessing it in strict mode will throw — Bergi
– Bergi, Commented Aug 14, 2022 at 22:33

CertainPerformance · Accepted Answer · 2022-08-14 21:46:18Z

2

Object.keys will only list enumerable properties. The arguments property is not enumerable, as you can see if you use getOwnPropertyDescriptor.

The property exists, but

function f() {

}
console.log(Object.getOwnPropertyDescriptor(f, 'arguments'));

To get a list of all own-property keys (excluding symbols), including non-enumerable ones, you can use Object.getOwnPropertyNames.

function f() {

}
console.log(Object.getOwnPropertyNames(f));

which, here, gives you

[
  "length",
  "name",
  "arguments",
  "caller",
  "prototype"
]

Object.keys could have returned properties if any properties put directly on the function were enumerable, such as

function f() {

}
f.someProp = 'foo';
console.log(Object.keys(f));

answered Aug 14, 2022 at 21:46

CertainPerformance

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