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Today we battled with a fearsome error, an error of odd implications and no clear solutions. Uncaught TypeError: Super expression must either be null or function
What does that even mean!? ?
Well, it means that when you import TransitionableComponent from 'react-transitionable-component', you get an object instead of a function. We confirmed the problem with some console.log calls. After importing TransitionableComponent is an object.
It looks just like a React component is supposed to. There’s a constructor method, a bunch of default object methods, and – I assume – all the Component methods as well. That’s great when you want to use a component in your render() function. Not so great when you want to use it as a parent class.
When you do something like class Arc extends TransitionableComponent, it fails. You can extend a null or a function, but not an object.
You can inherit from a class, but not from an instance of a class. I’m sure it’s like that in every language, but the reason it’s like that in JavaScript is that class TransitionableComponent extends Component transpiles into:
var TransitionableComponent = (function (_Component) {
_inherits(TransitionableComponent, _Component);
function TransitionableComponent(props) {
_classCallCheck(this, TransitionableComponent);
// I think this is super(props)
var _this = _possibleConstructorReturn(
this,
Object.getPrototypeOf(TransitionableComponent).call(this, props)
);
// this is where your constructor body goes
return _this;
}
_createClass(TransitionableComponent, [
{
// this is where your class body goes
},
]);
return TransitionableComponent;
})(_react.Component);
That _inherits call is the crucial piece. It does a bunch of .prototype magic to extend the definition of a given class with the definition of a child class. Instances don’t have prototypes, functions do.
Functions have a prototype property because of legacy reasons, I’m sure. That’s how JavaScript has always understood the concept of classes – generator/constructor functions double as classes.
At this point TransitionableComponent is a function - just like we’d expect. With some console.log-ing we confirmed that it remains a function right until the point we import it in our sample project.
Hmmm ?
I don’t know why it becomes an object. Our guessing and prodding didn’t reveal much. This is what importing transpiles to:
var _TransitionableComponent = __webpack_require__(16);
var _TransitionableComponent2 = _interopRequireDefault(
_TransitionableComponent
);
function _interopRequireDefault(obj) {
return obj && obj.__esModule ? obj : { default: obj };
}
This code might return an object, if you’re importing something that isn’t an ES6 Module. But we know that’s not our problem because TransitionableComponent.default is undefined.
You’d think __webpack_require__ was instantiating our component and returning an object instead of a function, but it works correctly when you import React’s default Component. Curiously, React’s compiled code looks like normal ES5 without even a hint of Webpack or Babel. Hmmm ?
I am at a loss. I have no idea what’s going on or why. But until we figure this out, react-transitionable-component will not be a usable library and my chance at open source glory lays trampled in the wastelands of npmjs.com. ?
Ideas?
PS: the edited and improved versions of these videos are becoming a video course. Readers of the engineer package of React+d3js ES6 get the video course for free when it’s ready.
Continue reading about Livecoding #20: You can’t extend an object
Semantically similar articles hand-picked by GPT-4
- Livecoding #21: Use Babel for libraries, not Webpack
- Livecoding #18: An abstract React transition component
- Livecoding #19: It’s hard to package a library
- Livecoding #27: New React Indie Bundle page almost done
- This is Yak Shaving
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