Google Apps Script and JavaScript Arrays. Method .reduce()

Short description of the method

The .reduce() method, like most other methods using the callback function, is a for loop, inside which the callback function sequentially processes all elements of the array. However, this is the only method that uses another parameter - accumulator, due to which the .reduce() method is often used to calculate the sum of the array.

The .reduce() method can pass 4 parameters to the callback function:

1. the current value of the sum of the element of the array accumulator
2. the current value of the array element item
3. optional parameter index - index of the current element
4. optional and rarely used parameter array

This method can also be used for more complex calculations. For example, below is a script that creates an array of indexes of the elements of the arr array, which are odd numbers.

const arr = [1, 2, 3, 4, 5];
  
// sum with for loop
  let sumArr = 0;
  for (let i = 0; i < arr.length; i++) {
    sumArr += arr[i];
  };
  console.log(sumArr);
  
// sum with forEach method
  let sumArr_1 = 0;
  arr.forEach(item => sumArr_1 += item);
  console.log(sumArr_1);
  
// sum with reduce method
  let sumArr_2 = arr.reduce((acc, item) => acc + item, 0);
  console.log(sumArr_2);
  
// with initial value
  sumArr_2 = arr.reduce((acc, item, index, array) => {
    console.log(`acc = ${acc}, item = ${item}, index = ${index}`);
    return acc + item;
  }, 0);
  console.log(sumArr_2);
  
// without initial value
  sumArr_2 = arr.reduce((acc, item, index, array) => {
    console.log(`acc = ${acc}, item = ${item}, index = ${index}`);
    return acc + item;
  });
  console.log(sumArr_2);
  
// find indexes items that are odd numbers  
  let newArr = arr.reduce((acc, item, index) => {
    if (item % 2 !== 0) acc.push(index);
    return acc;
  }, []);
  console.log(newArr); // [ 0, 2, 4 ]
  
 // reduceRight
  sumArr_2 = arr.reduceRight((acc, item, index, array) => {
    console.log(`acc = ${acc}, item = ${item}, index = ${index}`);
    return acc + item;
  });
  console.log(sumArr_2);

You can find more information in this video (RU voice):