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Objects, Functions, and Classes25% of the exam

Classes: inheritance, private and static members

How class syntax maps onto constructor functions and prototypes, and how to use extends and super, private # fields, accessors, static members and Error subclasses to meet a requirement, including the ReferenceError, TypeError and SyntaxError traps.

Official exam objective
Given a business requirement, apply fundamentals of class implementation to solve the business requirement.

~7 min read · 15 practice questions ·

What the exam expects

  • Explain what a class compiles to (a constructor function plus a prototype) and predict typeof, instanceof and prototype checks on classes and instances.
  • Know the behaviour changes when constructor functions move to classes: TDZ instead of hoisting, new required, strict mode, non-enumerable methods.
  • Write a subclass with extends, call super() before this, and reuse a parent method with super.method().
  • Choose between a #private field, a getter with no setter, a public field and a static member from an encapsulation or sharing requirement.
  • Predict this in static factories and detached class methods, and subclass Error for a custom error type.
  • Key terms: derived class (a class declared with extends); private element (a #name field or method readable only inside its class body); static member (defined on the class itself, not on instances); class field (a property declared in the class body and created on each instance); accessor (a get/set pair that runs code on read or write).

Key facts

typeof a class
Detail
'function'
Source
Classes (opens in a new tab)
Class before declaration
Detail
ReferenceError (temporal dead zone)
Source
class (opens in a new tab)
Class without new
Detail
TypeError
Source
Using classes (opens in a new tab)
Class body
Detail
Always strict mode
Source
class (opens in a new tab)
Class methods
Detail
On the prototype, shared, non-enumerable
Source
Method definitions (opens in a new tab)
this before super()
Detail
ReferenceError in a derived constructor
Source
super (opens in a new tab)
Default derived constructor
Detail
Passes all arguments to the parent constructor
Source
constructor (opens in a new tab)
#name outside the class
Detail
Early SyntaxError; script does not run
Source
Private elements (opens in a new tab)
Static members
Detail
On the class; not reachable through instances; inherited by subclasses
Source
static (opens in a new tab)
Public fields
Detail
Own, enumerable properties of each instance
Source
Public class fields (opens in a new tab)
instanceof
Detail
Searches the whole prototype chain
Source
instanceof (opens in a new tab)
Error subclass name
Detail
Inherited as "Error" unless set
Source
Error (opens in a new tab)
Sources for this section (11)

Check yourself

  • Why does `console.log(wallet.#balance)` outside the class stop even earlier lines from logging?
  • When does a derived constructor throw a ReferenceError?
  • Why does `new this()` inside an inherited static factory create a subclass instance?
  • What does `typeof` return for a class?
  • Which check returns true for an instance of any subclass of `Asset`?

The questions are free to read. Answers with their sources and self-grading are included with the pack.

Keep reading with the JavaScript Developer pack

  • Core concepts
  • Exam traps
  • Worked scenario

The pack includes:

  • All 24 lessons of the JavaScript Developer guide, with reading progress
  • Interactive decision walks and step-by-step sequence steppers
  • Inline practice questions and one-click topic practice (15 questions for this lesson)
  • An official source cited for every claim
  • An AI tutor that answers from the guide
  • Unlimited timed practice exams
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Practice for the JavaScript Developer exam