Callback Functions: Passing Functions as Arguments

// FIRST EXAMPLE //

function iReturnThings (thing) {
  return thing;
}

iReturnThings({ firstName: 'Brendan', lastName: 'Eich' });
// => {firstName: "Brendan", lastName: "Eich"}

iReturnThings(function () { return 4 + 5; });
// => ƒ () { return 4 + 5; }

// SECOND EXAMPLE //

function iInvokeThings (thing) {
  return thing();
}

iInvokeThings(function () { return 4 + 5; });
// => 9

iInvokeThings(function () { return 'Hello, ' + 'world!'; });
// => "Hello, world!"