How to prevent object/array mutation?

Question

I've been trying to debug weird issue and I've finally figured out why it's happening. Just not sure how to prevent it (; I have this function:

getInfo(id) {
  id = id || "zero";
  let i = routeDefinitions.findIndex(r => Boolean(r.name.toLowerCase().match(id)));
  // console.log(i) - works in plunker
  // but in my app sometimes returns -1...
  let current = routeDefinitions[i];
  let next = routeDefinitions[i + 1] ? routeDefinitions[i + 1] : false;
  let prev = routeDefinitions[i - 1] ? routeDefinitions[i - 1] : false;
  return { prev, current, next };
}

..it works perfectly in this plunker , but in my app I use its return value to update app state (custom implementation of redux pattern). When I send return value through this function:

  private _update(_old, _new) {
    let newState = Object.keys(_new)
      .map(key => {
        if (_old[key] === undefined) {
          _old[key] = _new[key];
        } else if (typeof _new[key] === "object") {
          this._update(_old[key], _new[key]);
        } else {
          _old[key] = _new[key];
        }
        return _old;
      })
      .find(Boolean);

    return Object.assign({}, newState || _old);
  }

.. routeDefinitions array is mutated and things start to break... I've tried couple of things:

let current = [...routeDefinitions][i];
// and:
return Object.assign({}, { prev, current, next });

..but it didn't work. How can I prevent mutatation of routeDefinitions array?

EDIT : I've managed to reproduce the error in this plunker

Answer 1

routeDefinitions array is mutated and things start to break

If your function is truly:

getInfo(id) {
  id = id || "zero";
  let i = routeDefinitions.findIndex(r => Boolean(r.name.toLowerCase().match(id)));
  // console.log(i) - works in plunker
  // but in my app sometimes returns -1...
  let current = routeDefinitions[i];
  let next = routeDefinitions[i + 1] ? routeDefinitions[i + 1] : false;
  let prev = routeDefinitions[i - 1] ? routeDefinitions[i - 1] : false;
  return { prev, current, next };
}

Then routeDefinitions is not mutated . Something else is mutating routeDefinitions.

Answer 2

I solved this by modifying _update() like this:

  private _update2(_old, _new) {
    let newState = {}; 
    Object.keys(_new)
      .map(key => {
        if (_old[key] === undefined) {
          newState[key] = _new[key];
        } else if (typeof _new[key] === "object") {
          newState[key] = this._update2(_old[key], _new[key]);
        } else {
          newState[key] = _new[key];
        }
        return newState;
      })
      .find(Boolean);

    return Object.assign({}, _old, newState);
  } 

I use old state just to check values, don't modify it until later when _update() is finished.

Plunker