setInterval用于运行if语句不起作用的函数

Question

I need to use the setInterval function to run the function change() after every 3 seconds but what I have tried so far is not working:

  var image = document.getElementById("gif1"); var image_tracker = '1gif' ; function change() { if(image_tracker =='1gif') { image.src='2.gif' ; image_tracker = '2gif' ; } else if (image_tracker ='2gif') { image.src='3.gif' ; image_tracker = '3gif' ; } else if (image_tracker = '3gif') { image.src='1gif' ; image_tracker = '1gif' ; } } var timer = setInterval ('change () ; ' , 3000 ) ; 

 <img src="1.gif" id="gif1" alt="1.gif" style="width:400px;height:400px;border:1px solid black;"> 

Answer 1

You have a bunch of syntax errors all over your code:

1. Your elseif conditions are assignment operators = and should be replaced with a comparison operator such as the loose equality operator == or the strict equality operator === depending on your requirement [Check this article for an in-depth explanation of the differences between the two].

2. You are assigning your setInterval() to a variable thus preventing it from running.

3. You are referencing a string called change() inside your setInterval() . Remove the quotes since change is a function.

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Check this jsFiddle or the Code Snippet below to see the gif change every 3 seconds:

 var img = document.getElementById("gif1"); var image_tracker = '1gif'; function change() { if(image_tracker == '1gif') { img.src = '//media.giphy.com/media/13gvXfEVlxQjDO/giphy.gif'; image_tracker = '2gif' ; } else if (image_tracker == '2gif') { img.src= '//upload.wikimedia.org/wikipedia/commons/2/2c/Rotating_earth_%28large%29.gif'; image_tracker = '3gif' ; } else if (image_tracker == '3gif') { img.src = '//colinbendell.cloudinary.com/image/upload/c_crop,f_auto,g_auto,h_350,w_400/v1512090971/Wizard-Clap-by-Markus-Magnusson.gif'; image_tracker = '1gif' ; } } setInterval(change , 3000 ) ; 

 <img src="//colinbendell.cloudinary.com/image/upload/c_crop,f_auto,g_auto,h_350,w_400/v1512090971/Wizard-Clap-by-Markus-Magnusson.gif" id="gif1" alt="1.gif" style="width:400px;height:400px;border:1px solid black;" />