For the usual count of an object’s own, enumerable string-keyed properties, use Object.keys(obj).length. Use a different API if you need to include non-enumerable properties, symbol keys, or inherited properties.
Choose what you mean by “properties”
JavaScript objects do not have a built-in length value for their properties. The right count depends on whether you mean own or inherited properties, enumerable or non-enumerable properties, and string keys or symbol keys.
| What you want to count | Code | Included keys |
|---|---|---|
| Usual object-data count | Object.keys(obj).length |
Own, enumerable string keys |
| All own string properties | Object.getOwnPropertyNames(obj).length |
Enumerable and non-enumerable own string keys |
| Every own property key | Reflect.ownKeys(obj).length |
Enumerable and non-enumerable own string and symbol keys |
| Enumerable strings on the object and its prototype chain | for...in |
Own and inherited enumerable string keys |
MDN’s Object.keys reference defines its result as an array of an object’s own enumerable string-keyed property names. Its property ownership and enumerability guide explains how the other methods differ.
Count the usual object properties
Use Object.keys() when you want the number of ordinary, enumerable string-keyed properties directly on the object:
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const user = { name: "Ava", active: true };
const count = Object.keys(user).length;
console.log(count); // 2Inherited properties are not included. Neither are non-enumerable properties or symbol-keyed properties. This makes Object.keys(obj).length a concise choice for typical data objects, but not a count of every possible property.
Include hidden or symbol-keyed properties
Use Object.getOwnPropertyNames() when non-enumerable string properties should count, or Reflect.ownKeys() when every own key—including symbols—should count:
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const obj = {};
Object.defineProperty(obj, "hidden", { value: 1 });
const id = Symbol("id");
obj[id] = 2;
obj.visible = 3;
Object.keys(obj).length; // 1: visible
Object.getOwnPropertyNames(obj).length; // 2: hidden, visible
Reflect.ownKeys(obj).length; // 3: hidden, visible, symbolObject.defineProperty() creates a non-enumerable property by default unless its descriptor sets enumerable: true. MDN documents Object.getOwnPropertyNames() as returning own string property names whether enumerable or not. Reflect.ownKeys() returns all own string and symbol keys, making it the single-call option for a complete own-key count.
Count inherited properties only when intended
A for...in loop visits enumerable string-keyed properties on the object and along its prototype chain. If you use it to count only the object’s own properties, check ownership before incrementing:
let count = 0;
for (const key in obj) {
if (Object.hasOwn(obj, key)) {
count++;
}
}Object.hasOwn() is available in modern JavaScript. For older environments, Object.prototype.hasOwnProperty.call(obj, key) is an alternative. The loop still excludes symbols and non-enumerable properties; use the APIs above when those need to count.
Arrays and other edge cases
Arrays
For ["x", "y"], Object.keys(array).length counts the two enumerable index properties. Reflect.ownKeys(array).length also includes the array’s non-enumerable length property. Choose the method based on whether structural properties such as length belong in your count.
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Null, undefined, and primitive values
Object.keys() throws a TypeError when passed null or undefined. Most other primitive values are coerced to objects; for example, strings expose enumerable index properties. If a function is intended to accept only objects, validate its input or document that requirement rather than assuming every argument is safe.
Property order
Order does not affect the count. For reference, Reflect.ownKeys() lists non-negative integer-index keys first in numeric order, then other string keys in creation order, followed by symbol keys in creation order.
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