javascript - 这两种函数调用方式的区别

Question

PopupFirstNameButton.addEventListener('click', FirstNameFunction);

// This gets called
function FirstNameFunction(){
  alert("Hello");   
}

// This does not
var FirstNameFunction = function (){
    alert("Hello");   
}

Answer 1

var FirstNameFunction = function (){
    alert("Hello");   
}

这是一个赋值语句，所以只有在它被执行之后，FirstNameFunction 才会被赋值。所以当 PopupFirstNameButton.addEventListener('click', FirstNameFunction);
正在执行，FirstNameFunction 是 undefined