I am trying to write this statement in simple javascript where import is not supported..
import IconAlignLeft from 'quill/assets/icons/align-left.svg';
That isn't even standard JavaScript. The use of import for things that are not JS modules is a feature of some bundlers (such as Webpack — relevant documentation is here).
There is no direct equivalent in plain JavaScript (in versions with or without support for modules).
The nearest equivalent would be:
const IconAlignLeft = "A string containing a URL that points to the SVG";
… but you would need to implement your own process for making the image available at the URL you specify.