Tests for ES6 class syntax "extends" On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS (new Base) instanceof Base PASS Object.getPrototypeOf(new Base):::Base.prototype PASS (new Derived) instanceof Derived PASS Object.getPrototypeOf(new Derived):::Derived.prototype PASS Object.getPrototypeOf(Derived.prototype):::Base.prototype PASS (new Derived).baseMethod():::"base" PASS (new Derived).overridenMethod():::"derived" PASS Derived.staticBaseMethod():::"base" PASS Derived.staticOverridenMethod():::"derived" PASS x = class extends:::SyntaxError: Unexpected end of script PASS x = class extends:::SyntaxError: Unexpected end of script PASS x = class extends Base {:::SyntaxError: Unexpected end of script PASS x = class extends Base { } PASS x = class extends Base { constructor() { } } PASS x.__proto__:::Base PASS Object.getPrototypeOf(x):::Base PASS x.prototype.__proto__:::Base.prototype PASS Object.getPrototypeOf(x.prototype):::Base.prototype PASS x = class extends null { constructor() { } }; x.__proto__:::Function.prototype PASS x.__proto__:::Function.prototype PASS x = class extends 3 { constructor() { } }; x.__proto__:::TypeError: The superclass is not a constructor. PASS x = class extends "abc" { constructor() { } }; x.__proto__:::TypeError: The superclass is not a constructor. PASS baseWithBadPrototype = function () {}; baseWithBadPrototype.prototype = 3; new baseWithBadPrototype PASS x = class extends baseWithBadPrototype { constructor() { } }:::TypeError: The value of the superclass's prototype property is not an object or null. PASS baseWithBadPrototype.prototype = "abc" PASS x = class extends baseWithBadPrototype { constructor() { } }:::TypeError: The value of the superclass's prototype property is not an object or null. PASS baseWithBadPrototype.prototype = null; x = class extends baseWithBadPrototype { constructor() { } } PASS x = 1; c = class extends ++x { constructor() { } };:::SyntaxError: Unexpected token '++' PASS x = 1; c = class extends x++ { constructor() { } };:::SyntaxError: Unexpected token '++'. Expected opening '{' at the start of a class body. PASS x = 1; c = class extends (++x) { constructor() { } };:::TypeError: The superclass is not a constructor. PASS x = 1; c = class extends (x++) { constructor() { } };:::TypeError: The superclass is not a constructor. PASS x = 1; try { c = class extends (++x) { constructor() { } } } catch (e) { }; x:::2 PASS x = 1; try { c = class extends (x++) { constructor() { } } } catch (e) { }; x:::2 PASS namespace = {}; namespace.A = class { }; namespace.B = class extends namespace.A { } PASS namespace = {}; namespace.A = class A { }; namespace.B = class B extends namespace.A { } PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends namespace.A { constructor() { } } PASS namespace = {}; namespace.A = class A { constructor() { } }; namespace.B = class B extends namespace.A { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends (namespace.A) { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends namespace["A"] { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; function getClassA() { return namespace.A }; namespace.B = class extends getClassA() { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; function getClass(prop) { return namespace[prop] }; namespace.B = class extends getClass("A") { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends (false||null||namespace.A) { constructor() { } } PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends false||null||namespace.A { constructor() { } }:::SyntaxError: Unexpected token '||'. Expected opening '{' at the start of a class body. PASS x = 1; namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends (x++, namespace.A) { constructor() { } }; PASS x = 1; namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends (namespace.A, x++) { constructor() { } };:::TypeError: The superclass is not a constructor. PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends new namespace.A { constructor() { } }:::TypeError: The superclass is not a constructor. PASS namespace = {}; namespace.A = class { constructor() { } }; namespace.B = class extends new namespace.A() { constructor() { } }:::TypeError: The superclass is not a constructor. PASS x = 1; namespace = {}; namespace.A = class { constructor() { } }; try { namespace.B = class extends (x++, namespace.A) { constructor() { } } } catch (e) { } x:::2 PASS x = 1; namespace = {}; namespace.A = class { constructor() { } }; try { namespace.B = class extends (namespace.A, x++) { constructor() { } } } catch (e) { } x:::2 PASS Object.getPrototypeOf((class { constructor () { } }).prototype):::Object.prototype PASS Object.getPrototypeOf((class extends null { constructor () { super(); } }).prototype):::null PASS new (class extends undefined { constructor () { this } }):::TypeError: The superclass is not a constructor. PASS x = undefined; new (class extends x { constructor () { super(); } }):::TypeError: The superclass is not a constructor. PASS class x {}; new (class extends null { constructor () { return new x; } }) instanceof x PASS new (class extends null { constructor () { this; } }):::ReferenceError: 'super()' must be called in derived constructor before accessing |this| or returning non-object. PASS new (class extends null { constructor () { super(); } }):::TypeError: function is not a constructor (evaluating 'super()') PASS x = {}; new (class extends null { constructor () { return x } }):::x PASS y = 12; class C extends null { constructor () { return y; } }; new C;:::TypeError: Cannot return a non-object type in the constructor of a derived class. PASS class x {}; new (class extends null { constructor () { return new x; } }) instanceof x PASS x = null; Object.getPrototypeOf((class extends x { }).prototype):::null PASS Object.prototype.isPrototypeOf(class { }) PASS Function.prototype.isPrototypeOf(class { }) PASS successfullyParsed TEST COMPLETE