在下面的代码中，何时调用/执行/调用JavaScript函数表达式？

let foo = function() {
  console.log("Hello, World!");
};

foo(); // 调用函数表达式

function doSomething(callback) {
  callback();
}

doSomething(function() {
  console.log("Callback function executed.");
});

(function() {
  console.log("I am a self-executing function.");
})();

document.getElementById("myButton").addEventListener("click", function() {
  console.log("Button clicked.");
});