Lodash _.keep() Method

The Lodash _.keep() method takes an array and a function and hence returns an array generated which keeps only true values based on the conditions of the function.

Syntax:

_.keep( array, function )

Parameters: This method takes two parameters as mentioned above and described below:

• array: The given array from which keep array is created.
• function: The function containing the conditions for elements to be kept.

Return Value: This method returns a newly created array.

Note: This will not work in normal JavaScript because it requires the lodash contrib library to be installed. 

Module Installation: Lodash contrib library can be installed using the following command:

npm install lodash-contrib –save

Example 1: In this example, we will create an array by keeping all positive values.

// Defining lodash contrib variable
var _ = require('lodash-contrib');

// Array
var array = [-1, -21, 43, 34, 12, -1];

// Getting keep array using keep() method
var k_array = _.keep(array, function(x) {
    if(x > 0) {
        return x;
    }
});

console.log("Original Array : ", array);
console.log("Generated keep Array : ", k_array);

Output:

Original Array :  [ -1, -21, 43, 34, 12, -1 ]
Generated keep Array :  [ 43, 34, 12 ]

Example 2: In this example, we will create an array by keeping all negative values.

// Defining lodash contrib variable
var _ = require('lodash-contrib');

// Array
var array = [-1, -21, -43, 34, 12, -1];

// Getting keep array using keep() method
var k_array = _.keep(array, function(x) {
    if(x < 0) {
        return x;
    }
});

console.log("Original Array : ", array);
console.log("Generated keep Array : ", k_array);

Output:

Original Array :  [ -1, -21, -43, 34, 12, -1 ]
Generated keep Array :  [ -1, -21, -43, -1 ]

Example 3: In this example, we will create an array by keeping all multiples of 2.

// Defining lodash contrib variable
var _ = require('lodash-contrib');

// Array
var array = [-1, -25, -43, 10, 125, -1];

// Getting keep array using keep() method
var k_array =_.keep(array, function(x) {
    if(x % 2 == 0) {
        return x;
    }
});

console.log("Original Array : ", array);
console.log("Generated keep Array : ", k_array);

Output: 

Original Array :  [ -1, -25, -43, 10, 125, -1 ]
Generated keep Array :  [ 10 ]