初学者JavaScript：更改if，else if或else进行切换

Question

Im in an intro JavaScript class and I have a homework assignment where I need to switch a picture every 5 seconds. I had it written out as a if, else if, else statement and it worked fine but then I saw on the directions that it's supposed to be a switch statement. Now I am having a tough time getting it to work. If anyone can help me out and let me know what Im doing wrong, that'd help a lot.

This is what I had:

var currAd = "pic1"; 
function changeAd() { 
if (currAd == "pic2") { 
document.images[0].src = "cvb1.gif"; 
currAd = "pic1"; 
} 
else if (currAd == "pic3") { 
document.images[0].src = "cvb2.gif"; 
currAd = "pic2"; 
} 
else { 
document.images[0].src = "cvb3.gif"; 
currAd = "pic3"; 
} 
}

This is the switch statement. I assume its not working because of the currAd variable being the same on all of the cases, but I dont know what to switch it to

var currAd; 
function changeAd() { 
switch (currAd) { 
case currAd: 
return document.images[0].src = "cvb2.gif"; 
case currAd: 
return document.images[0].src = "cvb3.gif"; 
default: 
return document.images[0].src = "cvb1.gif"; 
} 
}

Answer 1

Each case is the value you're comparing currAd with. You're currently comparing it to itself, when you should be comparing it to pic2 , pic3 , etc.

switch(curaAd) {
case "pic1":
  // ...

Answer 2

Change your switch statement to:

switch (currAd) { 
    case "pic1": 
        return document.images[0].src = "cvb2.gif"; 
    case "pic2": 
    return document.images[0].src = "cvb3.gif"; 
        default: 
    return document.images[0].src = "cvb1.gif"; 
}