How to call a function from another class in React-Native?

You have two options, either to use an object or use class name, let's start with object

class B {
  abc() {
    alert("Hello World");
  }
}
const b = new B();
export default b;

So when you call this file you can access the function abc like the following

import b from './B.js';
class A extends Component {
    _onItemPressed(item){
        b.abc();
    }
...

The other way is to use class instead as follow

class B{}
B.abc = function(){
    alert("Hello World");
}
module.exports = {
  functions: B
};

So when you call this file you can access the function abc like the following

import b from './B.js';
class A extends Component {
    _onItemPressed(item){
        b.functions.abc();
    }
...

Note: B class should not be a component, you can use it as a helper class.

also, you can enhance the way you deal with the object using singleton pattern as I already mention in React native- Best way to create singleton pattern

UPDATE: If you insist to use component instead of a class function, you can call it through reference, as follows:

export default class B extends Component {
  constructor(props) {
    super(props);
    this.abc = this.abc.bind(this);
   }
    abc(){
      alert('Hello World');
    }

    render() {
      return null
    }
}

now in A component you can call B by reference

import B from "./B.js";
class A extends Component {
  _onItemPressed(item) {
    this._b.abc();
  }
  render() {
    return (
      <TouchableHighlight
        underlayColor={Colors.colors.lightgrey}
        style={{ padding: 15 }}
        onPress={this._onItemPressed.bind(this)}
      >
        <Text>Click Me !</Text>
        <B ref={ref => (this._b = ref)} />
      </TouchableHighlight>
    );
  }
}

You dont initiate your class, to solve this you need to change the B.abc() to new B().abc();