TypeError: Class extends value undefined is not a function or null

I am getting the following error when trying to create these entities.

TypeError: Class extends value undefined is not a function or null

I am assuming this has something to do with circular dependencies, but how is that supposed to be avoided when using table inheritance and one to many relationships?

It is complaining about the following javascript at BaseComic_1.BaseComic.

let Variant = class Variant extends BaseComic_1.BaseComic {

Here is the complete file.

"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
    return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
Object.defineProperty(exports, "__esModule", { value: true });
const typeorm_1 = require("typeorm");
const Comic_1 = require("./Comic");
const BaseComic_1 = require("./BaseComic");
let Variant = class Variant extends BaseComic_1.BaseComic {
};
__decorate([
    typeorm_1.ManyToOne(type => Comic_1.Comic, comic => comic.variants),
    __metadata("design:type", Comic_1.Comic)
], Variant.prototype, "comic", void 0);
Variant = __decorate([
    typeorm_1.ClassEntityChild()
], Variant);
exports.Variant = Variant;
//# sourceMappingURL=Variant.js.map

---

import {Entity, Column, PrimaryGeneratedColumn, OneToMany} from "typeorm";
import {Comic} from "./Comic";

@Entity()
export class Series {

    @PrimaryGeneratedColumn()
    public id: number;

    @Column("text", {
        length: 30
    })
    public copyright: string;

    @Column("text", {
        length: 100
    })
    public attributionText: string;

    @Column("text", {
        length: 150
    })
    public attributionHTML: string;

    @Column("text", {
        length: 50
    })
    public etag: string;

    @Column("text", {
        length: 200
    })
    public title: string;

    @Column("text")
    public description: string;

    @Column("number", {
        length: 4
    })
    public startYear: number;

    @Column("number", {
        length: 4
    })
    public endYear: number;

    @Column("text", {
        length: 20
    })
    public rating: string;

    @Column("text", {
        length: 20
    })
    public type: string;

    @Column("text")
    public thumbnail: string;

    @OneToMany(type => Comic, comic => comic.series)
    public comics: Array<Comic>;
}

---

import {Entity, TableInheritance, PrimaryGeneratedColumn, Column, ManyToOne, DiscriminatorColumn} from "typeorm";
import {Series} from "./Series";

@Entity()
@TableInheritance("class-table")
@DiscriminatorColumn({ name: "type", type: "string"})
export class BaseComic {

    @PrimaryGeneratedColumn()
    public id: number;

    @Column("text", {
        length: 30
    })
    public copyright: string;

    @Column("text", {
        length: 100
    })
    public attributionText: string;

    @Column("text", {
        length: 150
    })
    public attributionHTML: string;

    @Column("text", {
        length: 50
    })
    public etag: string;

    @Column("text", {
        length: 200
    })
    public title: string;

    @Column("int")
    public issue: number;

    @Column("text")
    public variantDescription: string;

    @Column("boolean")
    public variant: boolean;

    @Column("text")
    public description: string;

    @Column("int")
    public pageCount: number;

    @Column("date")
    public onSaleDate: Date;

    @Column("date")
    public unlimitedDate: Date;

    @Column("text")
    public thumbnail: string;

    @ManyToOne(type => Series, series => series.comics)
    public series: Series;
}

---

import {OneToMany, ClassEntityChild} from "typeorm";
import {Variant} from "./Variant";
import {BaseComic} from "./BaseComic";

@ClassEntityChild()
export class Comic extends BaseComic {

    @OneToMany(type => Variant, variant => variant.comic)
    public variants: Variant[];
}

---

import {ManyToOne, ClassEntityChild} from "typeorm";
import {Comic} from "./Comic";
import {BaseComic} from "./BaseComic";

@ClassEntityChild()
export class Variant extends BaseComic {

    @ManyToOne(type => Comic, comic => comic.variants)
    public comic: Comic;
}

I was having the same issue. It turns out I was circularly importing classes, which is apparently a limitation. (See these GitHub issues: #20361, #4149, #10712)

Note that it seems that the circular reference is also limited between files, not simply types.

See this other answer