How does the prototype chain work in JavaScript?

Direct answer

When a property is not found as an own property, JavaScript follows the object’s internal [[Prototype]] link and repeats the lookup until it finds the property or reaches null.

JavaScript inheritance is based on delegation through prototypes. If obj.x is requested and x is not an own property of obj, lookup continues on Object.getPrototypeOf(obj) and then on that object’s prototype, continuing until null.

Methods such as array methods usually illustrate this well: an array instance does not contain its own copy of map. The method is found through Array.prototype in its prototype chain.

Object.hasOwn(obj, key) checks whether the property is owned directly by the object.
key in obj checks both own and inherited properties.
Assigning an own property with the same name can shadow an inherited property without modifying the prototype.