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!