umm... I just randomly checked and this person also seem to have packages which are of few lines. for e.g. I checked `is-fn`:

    export default function isFunction(value) {
      const type = Object.prototype.toString.call(value);

      return type === '[object Function]' ||
       type === '[object GeneratorFunction]' ||
       type === '[object AsyncFunction]';
     }

This has 120k weekly downloads: https://www.npmjs.com/package/is-fn

It's a polyfill for a buggy function, so it's not just arbitrary.

The real question is why JS can't provide the answer to "isFunction" in a builtin method

It can: JavaScript has a `typeof` operator that can be used for this. If you read the readme for is-fn, it points out that the package is only useful for code that needs to run in certain older browsers that have a buggy implementation of `typeof`.