gistfile1.js

function findArgs(acc, v, k) {
  if (typeof v === "string" && v[0] === '$')
    acc.push({ "path": k, "val": v.subst(1) });
  else if (Array.isArray(v) || typeof v === "object")
    acc = acc.concat(_.map(_.reduce(v, findArgs, []), (v) => { "path": k + "." + v.path, "val": v.val }));
  return acc;
}

function setField(path, val, into) {
  var c = path.split('.').reverse();
  while (c.length > 1) {
    into = into[c.pop()];
  }
  into[c.pop()] = val;
}

function extendType(functions, defaultTypeArgs) {
  var argPos = _.reduce(defaultTypeArgs, findArgs, []);
  return [function read(buffer, offset, typeArgs, context) {
    var args = _.cloneDeep(defaultTypeArgs);
    argPos.forEach((v) => {
      setField(v.path, typeArgs[v.val], args);
    });
    return functions[0](buffer, offset, args, context);
  }, function write(value, buffer, offset, typeArgs, context) {
    var args = _.cloneDeep(defaultTypeArgs);
    argPos.forEach((v) => {
      setField(v.path, typeArgs[v.val], args);
    });
    return functions[1](buffer, offset, args, context);
  }, function sizeOf(value, typeArgs, context) {
    var args = _.cloneDeep(defaultTypeArgs);
    argPos.forEach((v) => {
      setField(v.path, typeArgs[v.val], args);
    });
    if (typeof functions[2] === "function")
      return functions[2](buffer, offset, args, context);
    else
      return functions[2];
  }]
}