Notes for dynamic conversion of IMG. SRC

I have just written something in the past two days and need to dynamically convert the SRC of IMG. I take it for granted: If (IMG. src = '1.jpg ') IMG. the result of src = '2.jpg 'cannot be changed. The IMG can be found only when alert is used. SRC has a path, so IMG cannot be used. src = '1.jpg. So how to judge? Use string. indexof, because IMG. SRC is a string containing the path. We only need to find whether there is an image file name in this path. However, we need to make sure that the file name is case-insensitive before being determined.
Later I changed it to If (IMG. SRC. indexof ('1.jpg ')! =-1) IMG. src = '2.jpg.