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Lodash _.sumBy


The lodash method _.sumBy is a utility function that returns the sum of an array or object's numerical values. It iterates through each element in the collection, transforms the value using the provided iteratee function, and sums up the transformed values.

The function takes 2 parameters: the collection and the iteratee function. The collection can be either an array or an object. The iteratee function is optional and can be used to transform the values before they are summed up.

Here is the syntax for using _.sumBy:

_.sumBy(collection, [iteratee=_.identity])
  • collection: Array | Object - The collection to iterate over.
  • iteratee: Function - The function that accepts each value in the collection, transforms it, and returns the transformed value. This parameter is optional and defaults to _.identity which returns the original value unchanged.


  1. Summing up an array of numbers:
const numbers = [5, 10, 15];
const sum = _.sumBy(numbers);

// Output: 30
  1. Summing up an array of objects by a specific property:
const expenses = [
  { type: 'rent', amount: 1000 },
  { type: 'groceries', amount: 500 },
  { type: 'rent', amount: 750 },
  { type: 'gas', amount: 80 }
const rentExpenses = _.sumBy(expenses, (expense) => {
  if (expense.type === 'rent') {
    return expense.amount;
  return 0;

// Output: 1750
  1. Summing up values in an object:
const monthlyExpenses = {
  rent: 1000,
  groceries: 500,
  gas: 80

const sum = _.sumBy(monthlyExpenses);

// Output: 1580

In conclusion, _.sumBy is a simple and powerful utility function that makes it easy to sum up numerical values in arrays or objects. It's a handy tool for anyone working with collections of data in JavaScript.