CODEWITHSUNDEEP
node.jswebpackecmascript-6
JGleason
JGleason

Reputation: 3936

Why do I get "TypeError: Class extends value undefined is not a constructor or null" when trying to run webpack packaged node project

I have a simple application that is imported from webpack. The imported project exports a class like this...

export class BaseApp{...}

import { BaseApp } from "./MyClass"
import OtherThing from "./Other"
....
export { BaseApp, OtherThing }

Next I try to extend it in another node/express project...

import { BaseApp } from "@mine/util";

export class FullApp extends BaseApp{... }

I get...

class FullApp extends _mine_util__WEBPACK_IMPORTED_MODULE_0__["BaseA
pp"]{
                                                                         ^

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

How do I extend a class I am importing from Webpack?

Update here is an example project that demonstrates the issue...

https://github.com/jrgleason/simple-webpack-node

Upvotes: 5

Views: 14730

Answers (1)

MBer
MBer

Reputation: 2524

Impossible to say for certain when there is clearly much more to the source. If anyone else sees this error, the first thing to look for is circular dependencies: a file depends on some other file that (likely indirectly) depends on the first. JS must start somewhere, will not be able to ensure that a file is defined at the time that another file needs it, and will not try to go back and fill in the blanks later.

If you have more than a couple import / require statements, I recommend periodically running a checker like Madge to find and optionally visualize any loops before they become hard to undo.

npm i --saveDev madge
node node_modules/madge/bin/cli.js --warning --circular --extensions js ./

Upvotes: 12

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