JS Object Introspection Reference

Three ways to list keys — and when each is right

All three methods list properties of an object itself (not the prototype chain). The differences are about string vs symbol keys and enumerability.

Example

const sym = Symbol("id");
const o = {
  visible: 1,
  hidden: 2,
  [sym]: 3,
};
Object.defineProperty(o, "computed", { enumerable: false, value: 4 });

Object.keys(o);                     // ["visible", "hidden"]
Object.getOwnPropertyNames(o);      // ["visible", "hidden", "computed"]
Object.getOwnPropertySymbols(o);    // [Symbol(id)]
Reflect.ownKeys(o);                 // ["visible", "hidden", "computed", Symbol(id)]

Key ordering rules

All of these follow the same ordering: integer-like keys first, in ascending numeric order; then string keys in creation order; then symbols in creation order.

const k = { 10: "a", 2: "b", b: 1, a: 2 };
Object.keys(k); // ["2", "10", "b", "a"]  — note 2 before 10

Practical guidance

• Serializing data for JSON: Object.keys (matches JSON.stringify, which skips non-enumerable and symbol keys).
• Deep cloning or diffing: Reflect.ownKeys so hidden and symbol-keyed fields are not silently dropped.
• Inspecting library internals: getOwnPropertyNames, because private state is often stored in non-enumerable properties.

Prototype chain is never included

const base = { inherited: 1 };
const o = Object.create(base);
o.own = 2;
Object.keys(o);                // ["own"]
Object.keys(base);             // ["inherited"]

For a full chain walk combine with Object.getPrototypeOf.

See also

• Property descriptors
• Memory management