Node.js Tutorial - Node.js Underscore

Underscore (npm install underscore) is by far the most popular JavaScript library available on NPM.

It is the library with the largest number of dependents.

It is called underscore because it creates a global variable '_' when used in the browser.

In node, we can name the variable returned from require('underscore') whatever we want, but it is conventional to still do var _ = require('underscore').

Underscore provides a lot of functional programming support to JavaScript.


To filter elements which are greater than 100 in an array of numbers.

The function _.filter takes an array, passes each element of the array to a function (second argument), and returns an array containing all the elements where the second function returns true.

var foo = [1111, 10, 50, 200, 9, 90, 40]; 
var _ = require('underscore'); 
var results = _.filter(foo, function (item) { return item > 100 }); 

The function takes an array, calls a function for each element of the array storing the return value as a result, and returns a new array consisting of all the results.

It basically maps an input array, through a function, into an output array.

To multiply each element of an array with 2.


We can do that quite simply using as shown.

var foo = [1, 2, 3, 4]; 
var _ = require('underscore'); 
var results =, function (item) { return item * 2 }); 

To get all elements in a collections except those that match a condition.


For this, we can use _.reject.

An example to get only the odd elements in an array is shown as follows.

var _ = require('underscore'); 
var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; }); 
console.log(odds); // [1, 3, 5] 

To get the maximum element of an array use _.max, to get the minimum use _.min:

var _ = require('underscore'); 
var numbers = [10, 5, 100, 2, 1000]; 
console.log(_.min(numbers)); // 2 
console.log(_.max(numbers)); // 1000