Accumulate Complex Objects With the Reduce Method

Many people don’t realize the power and potential of the Array.reduce (MDN) method. What makes it so powerful is its ability to take in an array containing any values and then return almost any type of data. Because of this flexibility, I want to highlight one technique you can use to traverse an array containing any number of nested objects. You’ll learn how to accumulate all objects and return a single object you can use to reference any entry by ID.

Building out the function

First, we’ll start with the data, an array with a structure that looks like this:

const data = [
  {
    id: 1,
    children: [
      {
        id: 2,
        children: [
          {
            id: 3,
          },
          {
            id: 4,
            children: [
              {
                id: 5,
              },
            ],
          },
        ],
      },
      {
        id: 6,
        children: [{ id: 7 }],
      },
    ],
  },
];

The objective here is to turn this array of nested objects into a flattened object, so you can efficiently look up any entry by ID. Begin by creating the base function that accepts two arguments, list and accumulator. Right off the bat, passing the reduce accumulator around outside the reduce method might look a little funny. Bear with me; this is where all the magic happens!

function collect(list, acc) {
  // ...
}
collect(data, {});
// => undefined

Now take the list argument and call reduce to return a value other than undefined.

function collect(list, acc) {
  return list.reduce((accumulator, current) => {
    return accumulator;
  }, acc);
}
collect(data, {});
// => {}

Take note of line 4: when we recursively call the collect function, the resulting object is passed down and returned each time. Each iteration adds to the same object. Neat!

Next, you’ll set the object key to the current ID and assign it the value of current.

function collect(list, acc) {
  return list.reduce((accumulator, current) => {
    accumulator[current.id] = current;
    return accumulator;
  }, acc);
}
collect(data, {});
// => { '1': { id: 1, children: [ [Object], [Object] ] } }

You might call this finished if the object didn’t contain nested objects. In order to collect all those values too, you will need to recursively call collect if the current object contains the children property.

function collect(list, acc) {
  return list.reduce((accumulator, current) => {
    if (Array.isArray(current.children) && current.children.length) {
      return collect(current.children, accumulator);
    }
    accumulator[current.id] = current;
    return accumulator;
  }, acc);
}
collect(data, {});
// => { '1': { id: 1, children: [ [Object], [Object] ] },
//      '2': { id: 2, children: [ [Object], [Object] ] },
//      '3': { id: 3 },
//      '4': { id: 4, children: [ [Object] ] },
//      '5': { id: 5 },
//      '6': { id: 6, children: [ [Object] ] },
//      '7': { id: 7 } }

This is working great, but you can take this one step further and clean up the children property. You may want to do this so you have less duplicate data.

function collect(list, acc) {
  return list.reduce((accumulator, current) => {
    if (Array.isArray(current.children) && current.children.length) {
      accumulator[current.id] = {
        ...current,
        children: current.children.map((entry) => entry.id),
      };
      return collect(current.children, accumulator);
    }
    accumulator[current.id] = current;
    return accumulator;
  }, acc);
}
collect(data, {});
// => { '1': { id: 1, children: [ 2, 6 ] },
//      '2': { id: 2, children: [ 3, 4 ] },
//      '3': { id: 3 },
//      '4': { id: 4, children: [ 5 ] },
//      '5': { id: 5 },
//      '6': { id: 6, children: [ 7 ] },
//      '7': { id: 7 } }

Using Map Instead of an Object

Maybe you want to spice things up a bit and use the newer ES6 features like Map. Swap out any references to the accumulator object and replace that value with Map. Update the object assignments to use the set method, and that’s it!

function collect(list, acc) {
  return list.reduce((accumulator, current) => {
    if (Array.isArray(current.children) && current.children.length) {
      accumulator.set(current.id, {
        ...current,
        children: current.children.map((entry) => entry.id),
      });
      return collect(current.children, accumulator);
    }
    accumulator.set(current.id, current);
    return accumulator;
  }, acc);
}
collect(data, new Map());
// => Map {
//  1 => { id: 1, children: [ 2, 6 ] },
//  2 => { id: 2, children: [ 3, 4 ] },
//  3 => { id: 3 },
//  4 => { id: 4, children: [ 5 ] },
//  5 => { id: 5 },
//  6 => { id: 6, children: [ 7 ] },
//  7 => { id: 7 } }

Conclusion

There you have it, my thought process on how I would take a similar piece of data and transform it into something I can easily reference. Hopefully, there’s something here you can take away and apply to your own work. Happy coding!