How to get a functions's body as string?

If you're going to do something ugly, do it with regex:

A.toString().match(/function[^{]+\{([\s\S]*)\}$/)[1];

Don't use a regexp.

const getBody = (string) => string.substring(
  string.indexOf("{") + 1,
  string.lastIndexOf("}")
)

const f = () => { return 'yo' }
const g = function (some, params) { return 'hi' }
const h = () => "boom"

console.log(getBody(f.toString()))
console.log(getBody(g.toString()))
console.log(getBody(h.toString())) // fail !

You could just stringify the function and extract the body by removing everything else:

A.toString().replace(/^function\s*\S+\s*\([^)]*\)\s*\{|\}$/g, "");

However, there is no good reason to do that and toString actually doesn't work in all environments.