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Day 9 of #100DaysOfCode!

Christian Falucho
Passionate about building websites and web applications that can be used by millions of people. I hope to build tools that can help people.
・2 min read

Today's progress

Learned a little bit more about the reduce() method.

What I learned

The reduce()executes a provided function for each element in an array Reducing the array to a single value. The return value of the function is stored in accumulator (result/total).

For better explanation, let's go ahead dive into some code.

We have this array of numbers and inside the array we have all of these elements and we want to get the sum of all the elements. A typical for loop iterating over each element and adding them can solve this.

In addition, we can use the reduce() method to sum up all the elements in the array and returning a single value.

let numbers = [1, 2, 3, 4, 5]

let total = numbers.reduce(function(sum, number){
    return sum + number;

//output: 15
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In the code above we have the reduce() with a callback function that takes in two parameters. The first parameter being the sum (accumulator), which is the accumulated value. The second parameter, number is the currentValue being processed.

Here's a good example of what is happening under the hood.

let numbers = [1, 2, 3, 4, 5]

let total = numbers.reduce(function(sum, number){
    console.log("the accumulator is: " + sum, "the element's value is " + number)
    return sum + number;

//output: the accumulator is: 1 the current value is 2
//        the accumulator is: 3 the current value is 3
//        the accumulator is: 6 the current value is 4
//        the accumulator is: 10 the current value is 5
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Using the same code example as before but this time adding a console.log before our return statement. So we can visually see what is happening in each iteration.

Notice on the first iteration the accumulator is the value 1 and the second parameter current value is 2. Then on each iteration the accumulator becomes the sum of the previous accumulator and current value.

Filling in the gaps

The complete syntax for reduce() is as follows:

array.reduce(function(accumulator, currentValue, index, array){...})
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Simply put

The reduce() method helps to return a single value from an array, as a result of sequentially processing the results of the previous elements.

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