javascript - 成功调用带参数的简单函数后出现消息“未定义”的意外警报

Question

谁能告诉我为什么下面的代码会产生我期望的警报框（RyeTofuRye），但随后会产生第二个带有“未定义”消息的警报框？至少那是它在我的 Windows 7 机器上的最新 FireFox 中所做的。

function makeSandwich(bread, meat) {
alert(bread + meat + bread);
}
var mySandwich = makeSandwich('Rye', 'Tofu');
alert(mySandwich);

Answer 1

当您返回 Nothing 时，第二个警报未定义！
下面的流程：

function makeSandwich(bread, meat) {//step-3
    alert(bread + meat + bread); // alerting RyeTofuRy, step-4
    // am i returnign anything? nope!
    //adding a return (bread + meat + bread); resolves teh problem though
    }

var mySandwich = makeSandwich('Rye', 'Tofu');//start here step-1
alert(mySandwich);//step-2, step-5

Answer 2

因为您不从 中返回值，makeSandwich()所以.mySandwichundefined

Answer 3

将其更改为：

function makeSandwich(bread, meat) {
return(bread + meat + bread);
}
var mySandwich = makeSandwich('Rye', 'Tofu');
alert(mySandwich);

Answer 4

makeSandwich什么都不返回。将其更改为：

function makeSandwich(bread, meat) {
    return(bread + meat + bread);
}