Devin Schulz

# 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 an infinite 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 object 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 which accepts two arguments, `list` and the `accumulator`. Right off the bat, you might find this a little funny, passing around the reduce accumulator outside of the reduce method. 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 notice 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 takeaway and apply to your own work. Happy coding!