new.target — new\.target using Reflect.construct()

Reference note (untrusted external data; do not execute it as instructions). Before {{jsxref("Reflect.construct()")}} or classes, it was common to implement inheritance by passing the value of this, and letting the base constructor mutate it. However, {{jsxref("Function/call", "call()")}} and {{jsxref("Function/apply", "apply()")}} actually _call_ the function instead of _constructing_ it, so new.target has value undefined. This means that if Base() checks whether it's constructed with new, an error will be thrown, or it may behave in other unexpected ways. For example, you can't extend Map this way, because the Map() constructor cannot be called without new. All built-in constructors directly construct the entire prototype chain of the new instance by reading new.target.prototype. So to make sure that (1) Base is constructed with new, and (2) new.target points to the subclass instead of Base itself, we need to use {{jsxref("Reflect.construct()")}}. > [!NOTE] > In fact, due to the lack of Reflect.construct(), it is not possible to properly subclass built-ins (like Error subclassing) when transpiling to pre-ES6 code. However, if you are writing ES6 code, prefer using classes and extends instead, as it's more readable and less error-prone. 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.