These two functions are equivalent:

  const f1 = function(s) {
    let data;
    try {
      data = JSON.parse(s);
    } catch (err) {}

    if (data != null &&
        data.a != null &&
        data.a.b != null &&
        typeof data.a.b.c === 'string') {
      let x = parseFloat(data.a.b.c);
      if (x === x) {
        return x;
      }
    }
    return null;
  };

  const f2 =
  R.pipe(S.parseJson,
         R.chain(S.gets(['a', 'b', 'c'])),
         R.filter(R.is(String)),
         R.chain(S.parseFloat),
         S.fromMaybe(null));

Note that in the first function we have an unsafe expression,

  data.a.b.c

which we must guard with null/undefined checks. Unsafe expressions can easily get out of sync with their corresponding guards.

We can apply functional programming concepts to JavaScript code to obviate the need for null/undefined checks. The fact that we can't have all the benefits of FP in JS shouldn't dissuade us from taking advantage of the ideas which are applicable.