Difference between this ES6 arrow function and regular function? [duplicate]

Question

Still new to ES6 so trying to understand why there's a difference between these two functions below. I'm working in React and am noticing that I'm getting an error when writing a non-ES6 function that sets state. This is happening within componentDidMount.

This way in ES6 works and returns what i need:

(pos) => this.setState({
    lat: pos.coords.latitude,
    lng: pos.coords.longitude,
  })

However, to this way throws an error - "Uncaught TypeError: this.setState is not a function"

 function(pos) {
    this.setState({
      lat: pos.coords.latitude,
      lng: pos.coords.longitude
    })
  }

Aren't these the exact same thing? Can anyone explain why it would be throwing this error?

Here is the code from the react class to provide more context:

var GeolocationExample = React.createClass({
  getInitialState: function() {
    return {
      lat: '',
      lng: '',
    };
  },

  componentDidMount: function() {
    navigator.geolocation.getCurrentPosition(

      // Where I'm placing each of the above mentioned functions,

      (err) => alert(err.message),
    );
  },

  render: function() {
    return (
      <View>
        <Text>
          <Text style={styles.title}>Initial position: </Text>
          {this.state.lat}
        </Text>
        <Text>
          <Text style={styles.title}>Current position: </Text>
          {this.state.lng}
        </Text>
      </View>
    );
  }
});

Any and all help is appreciated. Thank you!

Answer 1

No they are not the same. Arrow functions are automatically bound to the context where they are created. That means that

(x) => this.stuff = x

is (mostly) equivalent to:

(function(x) {
    return this.stuff = x;
}.bind(this))

Arrow functions will also preserve the arguments, super and new.target of the function inside which it is created.

Which means

(function a() {
  const u = () => console.log(arguments);
  u("whatever");
})("a args");

will print something like ["a args"].

See here for more information.