javascript中的函数

function name(parameter1, parameter2, ... parameterN) {
  ...body...
}

function showMessage() {
  alert( 'Hello everyone!' );
}
showMessage();
showMessage();

function showMessage() {
  let message = "Hello, I'm JavaScript!"; // local variable
  alert( message );
}
showMessage(); // Hello, I'm JavaScript!
alert( message ); // <-- Error! The variable is local to the function

let userName = 'John';
function showMessage() {
  let message = 'Hello, ' + userName;
  alert(message);
}
showMessage(); // Hello, John