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.

Answer 2

It's essentially a no-operation function. It returns the value of whatever was passed into it.

The part about it being used as a "default iterator" within the library itself means that in other functions which may have an optional "iterator" parameter (which is likely used as a function to apply to each element of an array of some kind), if no iterator parameter is passed, the library will use this "no-op" iterator instead and the elements of the array will remain unchanged.

Answer 3

A specific example:

Underscore.js defines _.each and as like this.

_.each = function(obj, iterator, context) {
  ...
}

This iterator shows el value. You maybe have used this idiom.

_.each([1, 2, 3], function(el){
  console.log(el);
});

This iterator returns el value without change.

_.each([1, 2, 3], function(el){
  return el;
});

The function that returns a value without change occur frequently. So Underscore.js wants to define the function. Underscore.js names the function _.identity.

_.identity = function(value) {
  return value;
};

If Underscore.js wants to use a default iterator, all Underscore.js need is call _.identity.

_.each([1, 2, 3], _.identity);