这里是该语句的核心：

var monkey_love = prompt("Do you love the monkey?","Type yes or no");

if (monkey_love == "yes")

{

  alert("Welcome! I'm so glad you came! Please, read on!");

}

注意该条件中的两个等于标记，这是人们容易搞混的
地方之一。如果你只用一个标记，实际上是告诉
JavaScript测试是否monkey_love等于 "yes"。
幸运的是，多数浏览器在你运行这些语句时会识别
这些错误并警告你。但最好现在开始就注意别犯这
种错误

其他重要的条件是：

(variable_1 > variable_2)  is true if variable_1 is greater than variable_2

(variable_1 < variable_2)  is true if variable_1 is less than variable_2

(variable_2 <= variable_2)  is true if variable_1 is less than or equal to variable_2

(variable_1 != variable_2)  is true if variable_1 does not equal variable_2

在运行花括号中的语句前如果你想要两件事为“是”，
可这样做：

if ((variable_1 > 18) && (variable_1 < 21)) 

{ 

  document.writeln("variable_1 can vote, but can't drink.");

}

若想两件事之一为真，这样做：

if ((variable_1 == "bananas") || (variable_1 == "JavaScript")) 

{ 

  document.writeln("The monkey is happy because it has " +  variable_1);

}