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Lodash _.filter


The lodash _.filter method is a powerful utility function that allows developers to filter a collection using a custom predicate function. It can be used with arrays, objects, strings, and other iterable data types. The function loops through each element in the collection and executes a custom callback function on it. If the callback function returns true, the element is included in the filtered collection, otherwise it is excluded.

The basic syntax of the _.filter method is as follows:

_.filter(collection, [predicate=_.identity])
  • collection: The collection to iterate over. This can be an array, object, string, or any other iterable data type.

  • predicate: The function that is executed for each element in collection. If this argument is not provided or is undefined, then the function defaults to _.identity, which simply returns the truthy value of the element.

The predicate function takes three arguments:

  • value: The current element being processed in the collection.

  • index: The index of the current element in the collection.

  • collection: The collection that is being iterated over.

Here is an example of using the _.filter method to filter an array of numbers:

const numbers = [1, 2, 3, 4, 5];

const evenNumbers = _.filter(numbers, function(num) {
  return num % 2 === 0;

// Output: [2, 4]

In this example, the _.filter method loops through each number in the numbers array and executes the custom callback function on it. The function checks if the number is even by using the modulo operator, and returns true if it is. The evenNumbers array is then populated with only the even numbers from the original array.

The _.filter method also works with objects. Here is an example of filtering an object based on a property value:

const users = {
  'user1': { 'name': 'Alice', 'age': 25 },
  'user2': { 'name': 'Bob', 'age': 35 },
  'user3': { 'name': 'Charlie', 'age': 45 }

const seniors = _.filter(users, function(user) {
  return user.age >= 40;

// Output: [{ 'name': 'Charlie', 'age': 45 }]

In this example, the _.filter method loops through each object in the users object and executes the custom callback function on it. The function checks if the age property of the object is greater than or equal to 40, and returns true if it is. The seniors array is then populated with only the objects where the age property is 40 or above.

Overall, the _.filter method is a versatile and useful tool that allows developers to easily filter collections by custom criteria. By using this method, they can simplify their code and make it more readable and maintainable.