Is there any good example of use cases for angular.identity()?

Question

According to the documentation

A function that returns its first argument. This function is useful when writing code in the functional style.

I

Answer 1

angular.identity is useful for cases where you want to reference an object bound to a parent by passing it from a parent to its children.

An identity function is like the zero for functions. Kind of useless by itself, but occasionally useful as part of an expression using higher-order functions, where you can take a function as a parameter or return it as a result.

Here is an example for array values:

For example, you may bind a two-dimensional array to an initial selection, and then bind the contained inner arrays to each subselection. The values function in this case is the identity function: it is invoked for each group of child elements, being passed the data bound to the parent element, and returns this array of data.

Use it when it is necessary to pass a dummy function to:

• a promise ($q)
• a compiler ($compile)
• a unit test (spy/mock)

which acts as a data source or pump for a pipe|filter.

By comparison, Angular.noop incorrectly handles resolving/rejecting promises correctly because it always returns undefined, whereas Angular.identity correctly handles resolving/rejecting promises because it always returns the argument passed to it( f(x) = x ).

In terms of constructors in Angular, it is relevant as such:

JavaScript engines only return the instance of a constructor if the constructor does not explicitly return an object (i.e. a value of the type object or function). Hence new identity(value) is always an object. Thus new identity(value) == value only returns true if value is an object. This is because the equality operators in JavaScript always check for identity when one of the operands is an object.

References

• Purpose of an 'Identity Function'?

• What is Angular.noop used for?

• What is the practical use of the identity function in R?

• What do people use the identity function for?

• Use of the identity function in JavaScript

• Where and why is identity function useful?

• How do I pretty print an XSLT result document with removed source elements?

• d3 wiki: Selections

• AngularJS Issue #4579: orderBy filter doesn't support sorting by the value itself

• Promises and the danger of angular.noop

• The Identity Monad

• Function composition and the identity function

• Cocoa Drawing Guide: Transforms - The Identity Transform

• Understanding Fluent APIs

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

• https://softwareengineering.stackexchange.com/questions/256090/function-only-returns-unchanged-parameter-useless