We use many javascripts in a webpage such as banner sliding, facebook like, google plus, google analytics, vs... If we call all these javascript codes from one single .js file like the single line code below: <script src="scripts.js" type="text/javascript"></script> + does this have a good effect on SEO or no effect? + is there any disadvantage of using this method? I would like to know your comments and suggestions. Thanks.
This is a very good idea of using or handling a JS on your site. This really works in terms of SEO, because more JS the more loading time for your site. Less the JS less the loading time.
I think it may speed up your site to have just one js file since the client only has to download one file rather than many. Indirectly it may help seo if your pageload speeds are a bit faster. Some websites use this technique with images, where they will use one image with many icons on it and then use the image as a background image in a fixed position so that only the desire icon shows up. The reason for doing this is so that only one image is loaded which reduces pageloading speed.
It is a good method. If your site uses wordpress then there are some good plugins that consolidate and speed up Javascript loads.
This will reduce the DNS lookup times as well as the latency and a gain in loading speed. This may indirectly put a little positive effect on the SERPs. Its a good practice to merge JS files in single file.