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Introduction
Reduce is a method that can be difficult to understand especially with all the vague explanations that can be found on the web. There are a lot of benefits to understanding reduce
as it is often used in state management (think Redux).
The signature for the reduce
array method in JavaScript is:
arr.reduce(callback, initialValue);
Terminology
Reduce comes with some terminology such as reducer & accumulator. The accumulator
is the value that we end with and the reducer
is what action we will perform in order to get to one value.
You must remember that a reducer will only return one value and one value only hence the name reduce.
Take the following classic example:
const value = 0;
const numbers = [5, 10, 15];
for(let i = 0; i < numbers.length; i++) {
value += numbers[i];
}
The above will give us 30
(5 + 10 + 15). This works just fine, but we can do this with reduce
instead which will save us from mutating our value
variable.
The below code will also output 30
, but will not mutate our value
variable (which we have now called initialValue
)
/* this is our initial value i.e. the starting point*/
const initialValue = 0;
/* numbers array */
const numbers = [5, 10, 15];
/* reducer method that takes in the accumulator and next item */
const reducer = (accumulator, item) => {
return accumulator + item;
};
/* we give the reduce method our reducer function
and our initial value */
const total = numbers.reduce(reducer, initialValue)
The above code may look a little confusing, but under the hood there is no magic going on. Let’s add a console.log
in our reducer
method that will output the accumulator
and the item
arguments.
The following screenshot shows what’s logged to the console:
So the first thing we notice is our method is called 3
times because there are 3
values in our array. Our accumulator begins at 0
which is our initialValue
we passed to reduce
. On each call to the function the item
is added to the accumulator
. The final call to the method has the accumulator
value of 15
and item
is 15
, 15 + 15
gives us 30
which is our final value. Remember the reducer
method returns the accumulator
plus the item
.
So that is a simple example of how you would use reduce
, now let’s dive into more a complicated example.
Flattening an Array Using Reduce
Let’s say we have the following array:
const numArray = [1, 2, [3, 10, [11, 12]], [1, 2, [3, 4]], 5, 6];
And let’s say for some crazy reason, JavaScript has removed the .flat
method so we have to flatten this array ourselves.
So we’ll write a function to flatten any array no matter how deeply nested the arrays are:
function flattenArray(data) {
// our initial value this time is a blank array
const initialValue = [];
// call reduce on our data
return data.reduce((total, value) => {
// if the value is an array then recursively call reduce
// if the value is not an array then just concat our value
return total.concat(Array.isArray(value) ? flattenArray(value) : value);
}, initialValue);
}
If we pass our numArray
to this method and log the result we get the following:
This is a great example on how we can make a very common operation quite simple.
Let’s go over one more example.
Final Example – Changing an Object Structure
So with the new Pokemon game coming out, let’s pretend we have a server that sends us an array of Pokemon objects like so:
const pokemon = [
{ name: "charmander", type: "fire" },
{ name: "squirtle", type: "water" },
{ name: "bulbasaur", type: "grass" }
]
We want to change this object to look like:
const pokemonModified = {
charmander: { type: "fire" },
squirtle: { type: "water" },
bulbasaur: { type: "grass" }
};
To get to that desired output we do the following:
const getMapFromArray = data =>
data.reduce((acc, item) => {
// add object key to our object i.e. charmander: { type: 'water' }
acc[item.name] = { type: item.type };
return acc;
}, {});
If we call our method like so:
getMapFromArray(pokemon)
We get our desired output:
You can check out the Codesandbox here.
Conclusion
At first sight, the reduce
looks more complex than other JavaScript Array Iteration Methods like map
and filter
, but once the syntax, core concepts and use-cases are understood it can be another powerful tool for JavaScript developers.