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Lodash _.groupBy


The lodash _.groupBy method is a utility function that is used for grouping items in a collection based on a specified criterion or property. This method is commonly used in JavaScript applications to create data sets that can be easily analyzed or manipulated.

The _.groupBy method takes two parameters: the collection to be grouped and the criterion for grouping. The criterion can be a property name or a callback function that returns the group key for each item in the collection.

Here is an example usage of the _.groupBy method:

const scores = [
  { score: 70, subject: 'math' },
  { score: 85, subject: 'math' },
  { score: 90, subject: 'science' },
  { score: 80, subject: 'history' }

const groupedScores = _.groupBy(scores, 'subject');


// Output:
// {
//   'math': [ { score: 70, subject: 'math' }, { score: 85, subject: 'math' } ],
//   'science': [ { score: 90, subject: 'science' } ],
//   'history': [ { score: 80, subject: 'history' } ]
// }

In the example above, an array of scores is grouped by the subject property. The resulting object contains keys for each unique subject, with the corresponding values being arrays of all scores for that subject.

When using a callback function as the criterion, the function should return the group key for each item. Here is an example of grouping by a callback function:

const numbers = [1, 2, 3, 4, 5];
const groupedNumbers = _.groupBy(numbers, (num) => {
  return (num % 2 === 0) ? 'even' : 'odd';


// Output:
// {
//   'odd': [1, 3, 5],
//   'even': [2, 4]
// }

In this example, an array of numbers is grouped by whether they are odd or even, resulting in an object with keys of "odd" or "even", each containing the corresponding numbers from the original array.

Overall, the _.groupBy method is a powerful utility function that provides a simple and efficient way to group items in a collection based on a specified criterion.