html part of the code:
JS part of the code:
The effect that needs to be achieved: Click on the picture to switch back and forth.
The wrong way I started: The code looks like the picture above (logically it looks like there is no error)
Trying process: It is no problem to put the JS code before the closing tag of </body>, write it in the JS code file separately, import the JS text from the head tag, and report the error Uncaught TypeError: Cannot set property 'onclick' of null
Final solution: Put the JS code into window.load = function(){.........}, and introduce the JS text into the head tag, so it can run correctly. O(∩_∩)O
The second method: As mentioned in the above attempt, write the JS code before the </body> tag