javascript问题，为什么后两个结果的值是undefined呢？

Question

function speak() {
var greeting = "Hello, I'm " + identify.call( this );
console.log( greeting );
}
var me = {
name: "Kyle"
};
var you = {
name: "Reader"
};

alert(identify.call( me ));
alert(identify.call( you ));
alert(speak.call( me ));
alert(speak.call( you ));

其中alert(speak.call( me ));
alert(speak.call( you ));为什么显示的结果是undefined呢？？？、

Answer 1

因为你speek函数没有返回值

 function speak() {
var greeting = "Hello, I'm " + identify.call( this );
console.log( greeting );
return greeting;//增加返回值就不是undefined
}