Getting "Cannot call a class as a function" in my React Project

Question

I'm trying to add a React map component to my project but run into an error. I'm using Fullstack React's blog post as a reference. I tracked down where the error gets thrown in google_map.js line 83:

function _classCallCheck(instance, Constructor) { 
  if (!(instance instanceof Constructor)) { 
    throw new TypeError("Cannot call a class as a function"); 
    } 
  }

Here is my map component so far. The page loads just fine (without a map) when I comment out lines 58-60, the last three lines. edit: I made the changes that @Dmitriy Nevzorov suggested and it still gives me the same error.

import React from 'react'
import GoogleApiComponent from 'google-map-react'

export class LocationsContainer extends React.Component {
    constructor() {
        super()
    }
  render() {
    const style = {
        width: '100vw',
        height: '100vh'
    }
    return (
      <div style={style}>
        <Map google={this.props.google} />
      </div>
    )
  }
}

export class Map extends React.Component {
    componentDidUpdate(prevProps, prevState){
        if (prevProps.google !== this.props.google){
            this.loadMap();
        }
    }
    componentDidMount(){
        this.loadMap();
    }
    loadMap(){
        if (this.props && this.props.google){
            const {google} = this.props;
            const maps = google.maps;

            const mapRef = this.refs.map;
            const node = ReactDOM.findDOMNode(mapRef);

            let zoom = 14;
            let lat = 37.774929
            let lng = 122.419416
            const center = new maps.LatLng(lat, lng);
            const mapConfig = Object.assign({}, {
                center: center,
                zoom: zoom
            })
            this.map = new maps.Map(node, mapConfig)
        }
    }
    render() {
        return (
            <div ref='map'>
                Loading map...
            </div>
        )
    }
}

export default GoogleApiComponent({
  apiKey: MY_API_KEY
})(LocationsContainer)

And here is where this map component gets routed in main.js:

import {render} from 'react-dom';
import React from 'react';
import Artists from './components/Artists'
import { Router, Route, Link, browserHistory } from 'react-router'
import Home from './components/HomePage'
import Gallery from './components/ArtGallery'
import ArtistPage from './components/ArtistPage'
import FavsPage from './components/FavsPage'
import LocationsContainer from './components/Locations'

//Create the route configuration
render((
  <Router history={browserHistory}>
    <Route path="/" component={Home} />
        <Route path="locations" component={LocationsContainer} />
        <Route path="artists" component={Artists} /> 
        <Route path="gallery" component={Gallery} />     
      <Route path="favorites" component={FavsPage} />
      <Route path=":artistName" component={ArtistPage} />
  </Router>
), document.getElementById('app'))

Answer 1

For me it happened when I forgot to write extends React.Component at the end. I know it's not exactly what YOU had, but others reading this answer can benefit from this, hopefully.

Answer 2

For me it was because I forgot to use the new keyword when setting up Animated state.

eg:

fadeAnim: Animated.Value(0),

to

fadeAnim: new Animated.Value(0),

would fix it.

---

Edit from 5 years on with more explanation:

The most likely issue is that you're missing the new keyword somewhere, just like I did above. What this means is you don't even need to be using React to come across this error.

The issue is that in JS you can create classes like so (Example from MDN):

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }

  calcArea() {
    return this.height * this.width;
  }

}

If you wanted to use this class, you need to create a new instance of it like so:

const rect = new Rect(height, width);

The reason for this problem is often you're trying to do a function call to the definition of the class (or something inside the definition), rather than an instance of the class.

Essentially, in code, you're doing this:

Rectangle.calcArea() // incorrect!

when you should be doing

rect.calcArea() // correct!

Answer 3

tl;dr

If you use React Router v4 check your <Route/> component if you indeed use the component prop to pass your class based React component!

More generally: If your class seems ok, check if the code that calls it doesn't try to use it as a function.

Explanation

I got this error because I was using React Router v4 and I accidentally used the render prop instead of the component one in the <Route/> component to pass my component that was a class. This was a problem, because render expects (calls) a function, while component is the one that will work on React components.

So in this code:

<HashRouter>
    <Switch>
        <Route path="/" render={MyComponent} />
    </Switch>
</HashRouter>

The line containing the <Route/> component, should have been written like this:

<Route path="/" component={MyComponent} />

It is a shame, that they don't check it and give a usable error for such and easy to catch mistake.

Answer 4

Happened to me because I used

PropTypes.arrayOf(SomeClass)

instead of

PropTypes.arrayOf(PropTypes.instanceOf(SomeClass))

Answer 5

For me, it was ComponentName.prototype instead of ComponentName.propTypes. auto suggested by Phpstorm IDE. Hope it will help someone.

Answer 6

You have duplicated export default declaration. The first one get overridden by second one which is actually a function.

Answer 7

I experienced the same issue, it occurred because my ES6 component class was not extending React.Component.

Answer 8

Mostly these issues occur when you miss extending Component from react:

import React, {Component} from 'react'

export default class TimePicker extends Component {
    render() {
        return();     
    }
}

Answer 9

For me it was because i used prototype instead of propTypes

class MyComponent extends Component {

 render() {
    return <div>Test</div>;
  }
}

MyComponent.prototype = {

};

it ought to be

MyComponent.propTypes = {

};

Answer 10

Two things you can check is,

class Slider extends React.Component {
    // Your React Code
}

Slider.propTypes = {
    // accessibility: PropTypes.bool,
}

• Make sure that you extends React.Component
• Use propTypes instead of prototype (as per IDE intellisense)

Answer 11

Post.proptypes = {

}

to

Post.propTypes = {

}

someone should comment on how to monitor such error in a very precise way.

Answer 12

Looks like there're no single case when this error appears.

Happened to me when I didn't declare constructor in statefull component.

class MyComponent extends Component {

    render() {
        return <div>Test</div>;
    }
}

instead of

class MyComponent extends Component {

    constructor(props) {
        super(props);
    }

    render() {
        return <div>Test</div>;
    }
}

Answer 13

This is a general issue, and doesn't appear in a single case. But, the common problem in all the cases is that you forget to import a specific component (doesn't matter if it's either from a library that you installed or a custom made component that you created):

import {SomeClass} from 'some-library'

When you use it later, without importing it, the compiler thinks it's a function. Therefore, it breaks. This is a common example:

imports

...code...

and then somewhere inside your code

<Image {..some props} />

If you forgot to import the component <Image /> then the compiler will not complain like it does for other imports, but will break when it reaches your code.

Answer 14

In file MyComponent.js

export default class MyComponent extends React.Component {
...
}

I put some function related to that component:

export default class MyComponent extends React.Component {
...
}

export myFunction() {
...
}

and then in another file imported that function:

import myFunction from './MyComponent'
...
myFunction() // => bang! "Cannot call a class as a function"
...

Can you spot the problem?

I forgot the curly braces, and imported MyComponent under name myFunction!

So, the fix was:

import {myFunction} from './MyComponent'

Answer 15

I received this error by making small mistake. My error was exporting the class as a function instead of as a class. At the bottom of my class file I had:

export default InputField();

when it should have been:

export default InputField;

Answer 16

For me, it was because I'd accidentally deleted my render method !

I had a class with a componentWillReceiveProps method I didn't need anymore, immediately preceding a short render method. In my haste removing it, I accidentally removed the entire render method as well.

This was a PAIN to track down, as I was getting console errors pointing at comments in completely irrelevant files as being the "source" of the problem.

Answer 17

I had a similar problem I was calling the render method incorrectly

Gave an error:

render = () => {
    ...
}

instead of

correct:

render(){
    ...
}

Answer 18

I had it when I did so :

function foo() (...) export default foo

correctly:

export default  () =>(...);

or

const foo = ...
export default foo

Answer 19

For me it happened because I didn't wrap my connect function properly, and tried to export default two components

Answer 20

I faced this error when I imported the wrong class and referred to wrong store while using mobx in react-native.

I faced error in this snippet :

import { inject, Observer } from "mobx-react";

@inject ("counter")
@Observer

After few corrections like as below snippet. I resolved my issue like this way.

import { inject, observer } from "mobx-react";

@inject("counterStore")
@observer

What was actually wrong,I was using the wrong class instead of observer I used Observer and instead of counterStore I used counter. I solved my issue like this way.

Answer 21

I experienced this when writing an import statement wrong while importing a function, rather than a class. If removeMaterial is a function in another module:

Right:

import { removeMaterial } from './ClaimForm';

Wrong:

import removeMaterial from './ClaimForm';