Objectkeysobjlength

Objectkeysobjlength refers to the JavaScript expression Object.keys(obj).length, a common idiom for counting the number of own enumerable properties on an object. Object.keys(obj) returns an array containing the object's own property names that are enumerable, and the length property of that array gives the count.

What it counts and what it excludes: it counts only enumerable own properties directly on the object, not properties inherited from the prototype chain. It does not include non-enumerable properties, nor properties keyed by symbols. For arrays, Object.keys(array) returns the array indices that are enumerable as strings, so its length equals the number of enumerable properties, which may differ from array.length if holes or sparse elements exist.

Practical use and patterns: Object.keys(obj).length is widely used to determine the size of an object, to validate input, or to drive loops over an object’s properties. It can be combined with a loop like for (const key of Object.keys(obj)) to process each property.

Limitations and alternatives: since it only counts enumerable own properties, it will not reflect non-enumerable properties or symbol-keyed properties. If you need all own property names regardless of enumerability, use Object.getOwnPropertyNames(obj). To include symbols as well, combine Object.getOwnPropertyNames(obj) with Object.getOwnPropertySymbols(obj) or use Reflect.ownKeys(obj). For iterating key-value pairs, Object.entries(obj) is a related approach.

Example: const obj = {a:1, b:2, c:3}; Object.keys(obj).length yields 3.