demo/typescript.js
function getBaseConstructorTypeOfClass(type) {
            if (!type.resolvedBaseConstructorType) {
                var baseTypeNode = getBaseTypeNodeOfClass(type);
                if (!baseTypeNode) {
                    return type.resolvedBaseConstructorType = undefinedType;
                }
                if (!pushTypeResolution(type, 1 /* ResolvedBaseConstructorType */)) {
                    return unknownType;
                }
                var baseConstructorType = checkExpression(baseTypeNode.expression);
                if (baseConstructorType.flags & 80896 /* ObjectType */) {
                    // Resolving the members of a class requires us to resolve the base class of that class.
                    // We force resolution here such that we catch circularities now.
                    resolveStructuredTypeMembers(baseConstructorType);
                }
                if (!popTypeResolution()) {
                    error(type.symbol.valueDeclaration, ts.Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_base_expression, symbolToString(type.symbol));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                if (baseConstructorType !== unknownType && baseConstructorType !== nullType && !isConstructorType(baseConstructorType)) {
                    error(baseTypeNode.expression, ts.Diagnostics.Type_0_is_not_a_constructor_function_type, typeToString(baseConstructorType));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                type.resolvedBaseConstructorType = baseConstructorType;
            }
            return type.resolvedBaseConstructorType;
        }
function getBaseConstructorTypeOfClass(type) {
            if (!type.resolvedBaseConstructorType) {
                var baseTypeNode = getBaseTypeNodeOfClass(type);
                if (!baseTypeNode) {
                    return type.resolvedBaseConstructorType = undefinedType;
                }
                if (!pushTypeResolution(type, 1 /* ResolvedBaseConstructorType */)) {
                    return unknownType;
                }
                var baseConstructorType = checkExpression(baseTypeNode.expression);
                if (baseConstructorType.flags & 80896 /* ObjectType */) {
                    // Resolving the members of a class requires us to resolve the base class of that class.
                    // We force resolution here such that we catch circularities now.
                    resolveStructuredTypeMembers(baseConstructorType);
                }
                if (!popTypeResolution()) {
                    error(type.symbol.valueDeclaration, ts.Diagnostics._0_is_referenced_directly_or_indirectly_in_its_own_base_expression, symbolToString(type.symbol));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                if (baseConstructorType !== unknownType && baseConstructorType !== nullType && !isConstructorType(baseConstructorType)) {
                    error(baseTypeNode.expression, ts.Diagnostics.Type_0_is_not_a_constructor_function_type, typeToString(baseConstructorType));
                    return type.resolvedBaseConstructorType = unknownType;
                }
                type.resolvedBaseConstructorType = baseConstructorType;
            }
            return type.resolvedBaseConstructorType;
        }
The base constructor of a class can resolve to undefinedType if the class has no extends clause, unknownType if an error occurred during resolution of the extends expression, nullType if the extends expression is the null value, or an object type with at least one construct signature.