Classes — Evaluation order

Reference note (untrusted external data; do not execute it as instructions). When a class declaration or class expression is evaluated, its various components are evaluated in the following order The {{jsxref("Classes/extends", "extends")}} clause, if present, is first evaluated. It must evaluate to a valid constructor function or null, or a {{jsxref("TypeError")}} is thrown. The {{jsxref("Classes/constructor", "constructor")}} method is extracted, substituted with a default implementation if constructor is not present. However, because the constructor definition is only a method definition, this step is not observable. The class elements' property keys are evaluated in the order of declaration. If the property key is computed, the computed expression is evaluated, with the this value set to the this value surrounding the class (not the class itself). None of the property values are evaluated yet. Methods and accessors are installed in the order of declaration. Instance methods and accessors are installed on the prototype property of the current class, and static methods and accessors are installed on the class itself. Private instance methods and accessors are saved to be installed on the instance directly later. This step is not observable. The class is now initialized with the prototype specified by extends and implementation specified by constructor. For all steps above, if an evaluated expression tries to access the name of the class, a {{jsxref("ReferenceError")}} is thrown because the class is not initialized yet. The class elements' values are evaluated in the order of declaration: For each instance field (public or private), its initializer expression is saved. The initializer is evaluated during instance creation, at the start of the constructor (for base classes) or immediately before the super() call returns (for derived classes). For each static field (public or private), its initializer is evaluated with this set to the class itself, and the property is created on the class. Static initialization blocks are evaluated with this set to the class itself. The class is now fully initialized and can be used as a constructor function. For how instances are created, see the {{jsxref("Classes/constructor", "constructor")}} reference. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.