call-apply-method.js

// An object can be passed as the first argument to call or apply and this will be bound to it.
var obj = {a: 'Custom'};

// We declare a variable and the variable is assigned to the global window as its property.
var a = 'Global';

function whatsThis() {
  return this.a;  // The value of this is dependent on how the function is called
}

whatsThis(); // 'Global' as this in the function isn't set, so it defaults to the global/window object/globalThis

whatsThis.call(obj);  // 'Custom' as this in the function is set to obj
whatsThis.apply(obj); // 'Custom' as this in the function is set to obj

// .call() and .apply() method example 1
function add(c, d) {
    return this.a + this.b + c + d;
  }
  
  var o = {a: 1, b: 3};
  
  // The first parameter is the object to use as
  // 'this', subsequent parameters are passed as 
  // arguments in the function call
  add.call(o, 5, 7); // 16
  
  // The first parameter is the object to use as
  // 'this', the second is an array whose
  // members are used as the arguments in the function call
  add.apply(o, [10, 20]); // 34