callback1.js

//application/javascript
//
// Demonstrates callback from a class method
// By Andrew Kingdom
// MIT license

// Define a class
let MyClass1 = class {
  constructor(initialValue) {
    this.prop = initialValue;  // store the parameter value in a property
  }
  callback(fn) {
    let result = fn();
    console.log(result);
  }
}
// ... and do some things with it...

let b = new MyClass1( 1 ).prop;  // returns 1
console.log(b);

new MyClass1( 2 ).callback(function() {return 'hello';});  // returns "hello"

function message() { return 'hi' };  // define a function
new MyClass1( 3 ).callback(message);  // returns "hi"

console.log(this);  // returns [object Window]

new MyClass1( 4 ).callback(function() {return this;});  // returns [object Window], not the class instance

let a = new MyClass1( 5 );  // store an instance of the class

a.callback(function() {return this;});  // returns [object Window], not the class instance

a.callback(function() {return this.prop;});  // returns undefined -- no property named 'prop' in the Window object

a.callback(function() {return a.prop;});  // returns 5

a.prop = 8;  // change the property value
a.callback(function() {return a.prop;});  // returns 8