CLOSURES

The terms anonymous functions and closures are often incorrectly used interchangeably. Closures are functions that have access to variables from another function's scope. This is often accomplished by creating a function inside a function, as in the following highlighted lines from the previous createComparisonFunction() example:

function createComparisonFunction(propertyName) {         
 return function(object1, object2) {
  let value1 = object1[propertyName];
  let value2 = object2[propertyName];

  if (value1 < value2) {
   return -1;
  } else if (value1> value2) {
   return 1;
  } else {
   return 0;
  }
 };
}

The highlighted lines in this example are part of the inner function (an anonymous function) that is accessing a variable (propertyName) from the outer function. Even after the inner function has been returned and is being used elsewhere, it has access to that variable. This occurs because the inner function's scope chain includes the scope of createComparisonFunction...