objhasOwnProperty

objhasOwnProperty, commonly referred to as hasOwnProperty, is a method on JavaScript objects that tests whether a given property exists as an own property (not inherited through the prototype chain). It returns true if the property is directly present on the object, and false if the property is inherited or absent.

The typical usage is obj.hasOwnProperty(prop) where prop is a string or symbol naming the property. For example, var o = {a:1}; o.hasOwnProperty('a') is true, while o.hasOwnProperty('toString') is false because toString is inherited from Object.prototype. A common pattern in object enumeration is to filter keys with hasOwnProperty to avoid inherited properties, as in for (var k in o) { if (o.hasOwnProperty(k)) { ... } }. However, hasOwnProperty itself can be shadowed if an object defines its own property named hasOwnProperty, which would override the method from Object.prototype.

To guard against shadowing, a robust form is Object.prototype.hasOwnProperty.call(o, key). This ensures the check uses the original method regardless of the object’s own properties. It is also important to note that hasOwnProperty only checks own properties; properties on the object's prototype chain will not be reported as own properties.

As a modern alternative, ES2022 added Object.hasOwn(obj, prop), a static method that performs the same check without relying on an object's own hasOwnProperty method. This avoids the shadowing issue entirely and can be used as Object.hasOwn(o, 'prop').