Q:

Can I use a .js file for TSLint config? 

Choosing a right set of rules, especially for a team of more than 3 members takes a lot of discussion and time.  To understand some rules and the necessity of them in every single project members need to try the code with and without it, figuring out what works best. 
Thus, it would help a lot to be able to comment out and add hints comments to some rules in the tslint.json file but JSON doesn't allow comments.  
Even though TSLint seems to work fine with a JSON file with comments, it doesn't look right. 
Can I use a .js file, like I would do for other JS libs that support config files in JS/JSON? 

A:

Even though TSLint seems to work fine with a JSON file with comments, it doesn't look right. 

But it is supported for a reason.  Other tools in the TypeScript community also support it including the typescript compiler tsconfig.json.  
Summary
Use comments in json when its supported 

