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# Collection Functions
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Powerful iteration and data processing functions that work with arrays, objects, and strings. These functions form the core of functional programming patterns and data transformation workflows.
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## Capabilities
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### Map
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Creates an array of values by running each element in collection through iteratee.
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```javascript { .api }
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/**
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 * Creates an array of values by running each element in `collection` through
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 * `iteratee`. The `iteratee` is bound to `thisArg` and invoked with three
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 * arguments: (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [iteratee=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `iteratee`.
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 * @returns {Array} Returns the new mapped array.
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 */
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function map(collection, iteratee, thisArg);
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```
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**Usage Examples:**
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```javascript
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var map = require('lodash.map');
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// Transform numbers
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map([1, 2, 3, 4], function(n) { return n * 2; });
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// => [2, 4, 6, 8]
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// Extract property values
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var users = [{ name: 'alice' }, { name: 'bob' }];
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map(users, 'name');
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// => ['alice', 'bob']
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// Work with objects
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map({ a: 1, b: 2 }, function(value, key) { return key + value; });
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// => ['a1', 'b2']
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```
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### Filter
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Iterates over elements of collection, returning an array of all elements predicate returns truthy for.
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```javascript { .api }
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/**
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 * Iterates over elements of `collection`, returning an array of all elements
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 * `predicate` returns truthy for. The predicate is bound to `thisArg` and
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 * invoked with three arguments: (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [predicate=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `predicate`.
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 * @returns {Array} Returns the new filtered array.
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 */
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function filter(collection, predicate, thisArg);
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```
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**Usage Examples:**
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```javascript
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var filter = require('lodash.filter');
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// Filter numbers
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filter([1, 2, 3, 4, 5, 6], function(n) { return n % 2 === 0; });
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// => [2, 4, 6]
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// Filter objects by property
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var users = [
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  { name: 'alice', active: true },
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  { name: 'bob', active: false }
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];
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filter(users, { active: true });
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// => [{ name: 'alice', active: true }]
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filter(users, 'active');
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// => [{ name: 'alice', active: true }]
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```
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### Reduce
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Reduces collection to a value which is the accumulated result of running each element through iteratee.
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```javascript { .api }
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/**
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 * Reduces `collection` to a value which is the accumulated result of running
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 * each element in `collection` through `iteratee`, where each successive
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 * invocation is supplied the return value of the previous.
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function} [iteratee=_.identity] The function invoked per iteration.
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 * @param {*} [accumulator] The initial value.
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 * @param {*} [thisArg] The `this` binding of `iteratee`.
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 * @returns {*} Returns the accumulated value.
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 */
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function reduce(collection, iteratee, accumulator, thisArg);
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```
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**Usage Examples:**
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```javascript
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var reduce = require('lodash.reduce');
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// Sum numbers
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reduce([1, 2, 3], function(sum, n) { return sum + n; }, 0);
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// => 6
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// Group by property
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reduce([1.3, 2.1, 2.4], function(result, n) {
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  var key = Math.floor(n);
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  (result[key] || (result[key] = [])).push(n);
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  return result;
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}, {});
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// => { '1': [1.3], '2': [2.1, 2.4] }
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```
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### Find
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Iterates over elements of collection, returning the first element predicate returns truthy for.
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```javascript { .api }
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/**
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 * Iterates over elements of `collection`, returning the first element
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 * `predicate` returns truthy for. The predicate is bound to `thisArg` and
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 * invoked with three arguments: (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to search.
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 * @param {Function|Object|string} [predicate=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `predicate`.
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 * @returns {*} Returns the matched element, else `undefined`.
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 */
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function find(collection, predicate, thisArg);
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```
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**Usage Examples:**
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```javascript
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var find = require('lodash.find');
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var users = [
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  { name: 'alice', age: 25 },
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  { name: 'bob', age: 30 },
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  { name: 'charlie', age: 35 }
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];
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// Find by function
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find(users, function(user) { return user.age > 30; });
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// => { name: 'charlie', age: 35 }
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// Find by property match
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find(users, { name: 'bob' });
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// => { name: 'bob', age: 30 }
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// Find by property value
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find(users, 'name', 'alice');
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// => { name: 'alice', age: 25 }
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```
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### Each (forEach)
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Iterates over elements of collection invoking iteratee for each element.
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```javascript { .api }
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/**
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 * Iterates over elements of `collection` invoking `iteratee` for each element.
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 * The `iteratee` is bound to `thisArg` and invoked with three arguments:
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 * (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function} [iteratee=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `iteratee`.
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 * @returns {Array|Object|string} Returns `collection`.
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 */
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function forEach(collection, iteratee, thisArg);
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```
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### Every
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Checks if predicate returns truthy for all elements of collection.
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```javascript { .api }
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/**
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 * Checks if `predicate` returns truthy for **all** elements of `collection`.
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 * The predicate is bound to `thisArg` and invoked with three arguments:
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 * (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [predicate=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `predicate`.
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 * @returns {boolean} Returns `true` if all elements pass the predicate check, else `false`.
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 */
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function every(collection, predicate, thisArg);
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```
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### Some
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Checks if predicate returns truthy for any element of collection.
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```javascript { .api }
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/**
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 * Checks if `predicate` returns truthy for **any** element of `collection`.
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 * The predicate is bound to `thisArg` and invoked with three arguments:
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 * (value, index|key, collection).
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [predicate=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `predicate`.
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 * @returns {boolean} Returns `true` if any element passes the predicate check, else `false`.
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 */
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function some(collection, predicate, thisArg);
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```
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### GroupBy
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Creates an object composed of keys generated from the results of running each element through iteratee.
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```javascript { .api }
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/**
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 * Creates an object composed of keys generated from the results of running
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 * each element of `collection` through `iteratee`. The corresponding value
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 * of each key is an array of the elements responsible for generating the key.
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [iteratee=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `iteratee`.
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 * @returns {Object} Returns the composed aggregate object.
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 */
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function groupBy(collection, iteratee, thisArg);
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```
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**Usage Examples:**
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```javascript
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var groupBy = require('lodash.groupby');
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// Group by length
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groupBy(['one', 'two', 'three'], 'length');
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// => { '3': ['one', 'two'], '5': ['three'] }
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// Group by Math.floor
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groupBy([6.1, 4.2, 6.3], function(n) { return Math.floor(n); });
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// => { '4': [4.2], '6': [6.1, 6.3] }
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```
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### SortBy
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Creates an array of elements, sorted in ascending order by the results of running each element through iteratee.
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```javascript { .api }
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/**
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 * Creates an array of elements, sorted in ascending order by the results of
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 * running each element in a collection through `iteratee`. This method performs
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 * a stable sort, that is, it preserves the original sort order of equal elements.
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 *
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 * @param {Array|Object|string} collection The collection to iterate over.
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 * @param {Function|Object|string} [iteratee=_.identity] The function invoked per iteration.
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 * @param {*} [thisArg] The `this` binding of `iteratee`.
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 * @returns {Array} Returns the new sorted array.
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 */
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function sortBy(collection, iteratee, thisArg);
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```
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**Usage Examples:**
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```javascript
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var sortBy = require('lodash.sortby');
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var users = [
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  { name: 'charlie', age: 40 },
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  { name: 'alice', age: 30 },
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  { name: 'bob', age: 35 }
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];
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// Sort by property
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sortBy(users, 'age');
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// => [{ name: 'alice', age: 30 }, { name: 'bob', age: 35 }, { name: 'charlie', age: 40 }]
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// Sort by function
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sortBy(users, function(user) { return user.name; });
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// => [{ name: 'alice', age: 30 }, { name: 'bob', age: 35 }, { name: 'charlie', age: 40 }]
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```
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## Additional Functions
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The collection category also includes: `at`, `countBy`, `findLast`, `findWhere`, `forEachRight`, `includes`, `indexBy`, `invoke`, `max`, `min`, `partition`, `pluck`, `reduceRight`, `reject`, `sample`, `shuffle`, `size`, `where`
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**Package Installation:**
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```bash
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npm install lodash.map lodash.filter lodash.reduce lodash.find
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npm install lodash.foreach lodash.every lodash.some lodash.groupby
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npm install lodash.sortby
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```