Please explain usage of _.identity(value) of underscore.js

Question

Please explain usage of _.identity(value) of underscore.js. Not able to understand it from the documentation ( http://underscorejs.org/#identity ).

Can

Answer 1

A JavaScript code pattern involving identity is filtering values based on truthiness, e.g.

var a = [null, null, [1,2,3], null, [10, 12], null];

a.filter(_.identity) 

yields [Array[3], Array[2]].

Using

_.compact(a)

is clearer, but one may not use lodash or underscore at all, e.g.

function identity(x) { 
   return x; 
}

a.filter(identity)

Whether it is a good code pattern is questionable for several reasons, but it's being used in the wild.

It is not a NOOP at all. A NOOP is an imperative construct in e.g. assembly, whereas in functional programming, it is like other functions in that it returns a value. If identity were a NOOP, then all pure functions could also be considered noop, and it would not be a sensible thing.