".route" ? (from Eloquent Javascript) Is this some Javascript method or ES6 notation?

JavaScript
1

Hello, everyone. I’m working through Eloquent JavaScript and I got really, really stuck on a problem so I gave in and looked at the answer but there’s something in the answer I don’t understand.

First, the answer:

function lazyRobot({place, parcels}, route) {
  if (route.length == 0) {
    // Describe a route for every parcel
    let routes = parcels.map(parcel => {
      if (parcel.place != place) {
        return {route: findRoute(roadGraph, place, parcel.place),
                pickUp: true};
      } else {
        return {route: findRoute(roadGraph, place, parcel.address),
                pickUp: false};
      }
    });

    // This determines the precedence a route gets when choosing.
    // Route length counts negatively, routes that pick up a package
    // get a small bonus.
    function score({route, pickUp}) {
      return (pickUp ? 0.5 : 0) - route.length;
    }
    route = routes.reduce((a, b) => score(a) > score(b) ? a : b).route;
  }

  return {direction: route[0], memory: route.slice(1)};
}

runRobotAnimation(VillageState.random(), lazyRobot, []);

The part I don’t understand is .route after the end of the reduce function. Can anyone help me figure out what this does? Thank you!

Edit: Here’s the findRoute function for reference

function findRoute(graph, from, to) {
  let work = [{at: from, route: []}];
  for (let i = 0; i < work.length; i++) {
    let {at, route} = work[i];
    for (let place of graph[at]) {
      if (place == to) return route.concat(place);
      if (!work.some(w => w.at == place)) {
        work.push({at: place, route: route.concat(place)});
      }
    }
  }
}
2

so the .route you refer to is a property on the reduced array. As (I believe) the routes array is reduced to a single member, that member has a .route property. And the variable route is set to that property value.

Which chapter, and what context? With the bit of code you’ve supplied, it feels like there’s a lot of context missing.

3

Well, it’s one of the exercises and there’s a lot of functions at play but it’s from Chapter 7. http://eloquentjavascript.net/07_robot.html

4

Right, ok. so in the findRoute() you provide (I had to skim the chapter for context), the work array is being created. In that, each member contains an object that looks like:

{
  at:       place,
  route:  []  // route is an array, indicating the path to get to this place.
}

Thus, when the reduced is called, based on the comparitive values of score(a) and score(b), the final remaining member of that array (reduce is returning a single entity), contains those same two properties: at and route.

So, the line

route = routes.reduce((a, b) => score(a) > score(b) ? a : b).route;

is saying “set route equal to the route property of the single member returned by this reduce function”.

1 Like
5

Thanks for that very clear explanation. I still need to do some thinking about my own code (which I didn’t post) and why it’s not working, but I definitely feel clear about what “.route” is doing there. Thank you very much!

1 Like
6

Glad I could help, and best of luck!

1 Like