Understanding JavaScript reduce method

Posted on January 08, 2021


The JavaScript reduce() is a method of Array class that you can call on any arrays that you have in your JavaScript code. The method will execute a reducer function that you need to provide as the first argument.

But what’s a reducer function? It is simply a function that gradually accumulates your array values, combining them into a single value.

Understanding reduce() in theory is hard, so let me show you a very basic example of using the method.

Reduce use case: sum of array values

You need to find the sum of all the values in a single array. Here’s the array:

let scores = [3, 4, 8];

You need to find the sum of these three numbers, which is 16. Here’s how you do it with the reduce method:

let scores = [3, 4, 8];

let sum = scores.reduce(function (accumulator, currentValue) {
  console.log(`accumulator is ${accumulator} 
  and currentValue is ${currentValue}`);

  return accumulator + currentValue;
});

When you run the code above on the browser or Node, you will have the following console logs:

accumulator is 3
  and currentValue is 4
accumulator is 7
  and currentValue is 8

First, the accumulator is 3 because that’s the value of index 0 in your array. You can also provide the initial value for the accumulator by passing it as an argument after the callback/ reducer function.

Notice the initialValue variable being passed after the callback function below:

let scores = [3, 4, 8];
let initialValue = 0;

let sum = scores.reduce(function (accumulator, currentValue) {
  console.log(`accumulator is ${accumulator} 
  and currentValue is ${currentValue}`);

  return accumulator + currentValue;
}, initialValue);

Your console will now log the following records:

accumulator is 0
  and currentValue is 3
accumulator is 3
  and currentValue is 4
accumulator is 7
  and currentValue is 8

Summing it all up, the reduce method simply iterates through your array in order to execute the reducer function that you passed as a callback into it.

The signature of reduce method is as follows. I will explain it afterward:

array.reduce(reducerFunction, initialValue);

function reducerFunction(accumulator, currentValue, index, array) {
  // what you do here is up to you
  // but you must have a return statement
}

The reduce method will pass four arguments into your reducer function:

  • accumulator - The return value of the last execution. It can be the initial value or the value at index 0 depending on the code you write.
  • currentValue - The value of the current iteration. Starts from index 0 to the last index.
  • index - Optional. The index of the current iteration. You may need this to perform some task
  • array - Optional. The original array where you call reduce()

The last two parameters are optional, so you may include them only when you need to. The reducer function will be called as many times as the length of your array, with the accumulator value updated each time.

Finding maximum or minimum number with reduce

You can use the reduce method to find the highest or the lowest number value in an array:

let scores = [7, 6, 5];
let maxValue = scores.reduce(function (a, b) {
  return Math.max(a, b);
});

let minValue = scores.reduce(function (a, b) {
  return Math.min(a, b);
});

Removing duplicate values with reduce

You can also remove duplicate values of an array with reduce(). But since you want to return an array instead of a number, you need to define the initial value as an empty array. Take a look at the following code example:

let myScores = [1, 2, 7, 2, 5, 8, 7];
let initialValue = [];

let noDupeArray = myScores.reduce(function (accumulator, currentValue) {
  if (!accumulator.includes(currentValue)) {
    accumulator.push(currentValue);
  }
  return accumulator;
}, initialValue);

The accumulator starts as an empty array. If the current value is not yet included in the accumulator, you push the value into it.

The reduce method can return any data type that you want to return, as long as it’s a single value (a string, a number, an array or an object, or even boolean).

See also: how to sum a property in an array of objects

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