undefined - JavaScript | MDN
The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types.| MDN Web Docs
The TypeError object represents an error when an operation could not be performed, typically (but not exclusively) when a value is not of the expected type.| developer.mozilla.org
Symbol is a built-in object whose constructor returns a symbol primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique. Symbols are often used to add unique property keys to an object that won't collide with keys any other code might add to the object, and which are hidden from any mechanisms other code will typically use to access the object. That enables a form of weak encapsulation, or a weak form of information hiding.| developer.mozilla.org
We are an open community of developers, technical writers, and learners building resources for a better Web, regardless of brand, browser, or platform. Anyone can contribute, and each person who does contribute has an impact on millions of readers. Learn how to contribute and drive innovation on the Open Web.| developer.mozilla.org
The spread (...) syntax allows an iterable, such as an array or string, to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected. In an object literal, the spread syntax enumerates the properties of an object and adds the key-value pairs to the object being created.| developer.mozilla.org
A deep copy of an object is a copy whose properties do not share the same references (point to the same underlying values) as those of the source object from which the copy was made. As a result, when you change either the source or the copy, you can be assured you're not causing the other object to change too. That behavior contrasts with the behavior of a shallow copy, in which changes to nested properties in the source or the copy may cause the other object to change too.| developer.mozilla.org
The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types.| MDN Web Docs
The Object.defineProperty() static method defines a new property directly on an object, or modifies an existing property on an object, and returns the object.| MDN Web Docs
The set syntax binds an object property to a function to be called when there is an attempt to set that property. It can also be used in classes.| MDN Web Docs
The get syntax binds an object property to a function that will be called when that property is looked up. It can also be used in classes.| MDN Web Docs
Introduction This Ecma Standard defines the ECMAScript 2026 Language. It is the seventeenth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It| tc39.es
