Underscore.js Interview Questions & Answers (2025)

Underscore.js is a javascript library that provides helper functions to manipulate an object without extending code any core JavaScript objects.It helps us to write cleaner code which is easy to read and understand. Underscore.js is written by Jeremy Ashkenas and is an open-source component of DocumentCloud.

Collection Functions are functions that work on arrays, objects, and array-like objects such as arguments, NodeList and similar.Below is the list of some collection functions in Underscore.js.

Function name     Prototype
each         :  _.each(list, iteratee, [context])
map          :  _.map(list, iteratee, [context])
find         :  _.find(list, predicate, [context])
filter       :  _.filter(list, predicate, [context]) 
reject       : _.reject(list, predicate, [context])

Readmore from http://underscorejs.org/#collections

_.contains method is used to check an element in array exists or not in Underscore.js.

Prototype : _.contains(list, value, [fromIndex])

It will take three arguments first two are required and third is optional. The internally _.contains method uses indexOf if the supposed list is a type of array.

Example

var arrayList=['aaa', 'bbb', 'cfp', 'ddd'];
var needle1="cfp";
var needle2="abc";
_.contains(arrayList, needle1); // outputs true
_.contains(arrayList, needle2); // outputs false