Underscore.js

Underscore is a JavaScript library that provides a whole mess of useful functional programming helpers without extending any built-in objects.

Check the Underscore page here.

Why you need that?

Like a developer is good to know every tool available. In some cases you can use the flexibility of the Underscore library to improve your code.

Like this example:

--- > object_.object(list, [values]) Converts arrays into objects. Pass either a single list of [key, value] pairs, or a list of keys, and a list of values. If duplicate keys exist, the last value wins.

_.object(['moe', 'larry', 'curly'], [30, 40, 50]);
=> {moe: 30, larry: 40, curly: 50}

_.object([['moe', 30], ['larry', 40], ['curly', 50]]);
=> {moe: 30, larry: 40, curly: 50}

---> isMatch_.isMatch(object, properties)

Tells you if the keys and values in properties are contained in object.

var stooge = {name: 'moe', age: 32};
_.isMatch(stooge, {age: 32});
=> true

On my opinion, is a good idea to be familiar with this library since it provides tools to make your job easier. Hope this helps.

P.S: You can check all the tools on the oficial Underscore website here.

If you want to check the diferrence between a library and a framework, please Check Here.