This document discusses memory efficiency with closures in Node.js. It defines closures and describes three common use cases: 1) completion handlers where memory is retained until the handler returns; 2) intermediary functions where memory is retained until references are cleared; and 3) listener functions where memory is retained until listeners are removed. It provides examples of each use case and describes how understanding closure semantics is important for developing efficient Node.js code. The document warns that closures can lead to memory leaks if not properly managed and cleared.

1. Memory efficiency with Closures
Gireesh Punathil, Software Engineer, IBM
gpunathi@in.ibm.com | @gireeshpunam | https://github.com/gireeshpunathil

2. Agenda
 Introduction to Closures
 Different Closure Semantics
 Closure use cases
 Real world memory leak example
 Summary

3. Example of a Closure

4. Ways to use Closures
Closure
Function passed
as an argument
Function defined in-line in the
asynchronous call
Function defined in the lexical
scope, and passed as argument
to asynchronous call
Function defined in-line as a
listener to an event
Function defined in the lexical
scope, and name passed as
listener to events
Function
retrieved as a
return value
Function defined in the lexical
scope of an outer function, and
returned to the caller
#1
#3
#2

5. Use case #1: Completion handler
Memory allocation: Closure context is
created when completion handler is
defined
Memory retention: Closure and its
associated data is retained until:
 The completion handler returns
 Timer is cleared

6. Use case #1: Completion handler
Before the timer fires
After the timer fires

7. Use case #2: Intermediary function
Memory allocation: Closure context is
created when the inner function is defined
Memory retention: Closure and its
associated data is retained until:
The reference is manually cleared

8. Use case #2: Intermediary function
Even after the Closure is used up
After manual cleanup: data = null;

9. Use case #3: Listener functions
Memory allocation: Closure context is
created when the inner function is defined
Memory retention: Closure and its
associated data is retained until:
The listener is manually removed

10. Use case #3: Listener functions
Long after the run() returns
After removing listener: ev.removeAllListeners();

11. Real-world leak scenario

12. Summary
 Closure is a powerful semantic feature in functional programs
 Closure is pervasively used in Node / JavaScript
 Closure ‘remembers’ the environment in which it was created
 This remembrance can cost an inefficient program a lot of memory
 Closures in Node cover five discrete semantics
 In some cases the context is cleared automatically, in some, manually
 Understanding Closures closely is vital to developing efficient node code

13. Disclaimer
 The information contained in this presentation is provided for informational purposes only.
 While efforts were made to verify the completeness and accuracy of the information contained in this presentation, it is
provided “As Is”, without warranty of any kind, express or implied.
 In addition, this information is based on IBM’s current product plans and strategy, which are subject to change by IBM
without notice.
 IBM shall not be responsible for any damages arising out of the use of, or otherwise related to, this presentation or any
other documentation.
 Nothing contained in this presentation is intended to, or shall have the effect of:
 Creating any warranty or representation from IBM (or its affiliates or its or their supplies and/or licensors); or
 Altering the terms and conditions of the applicable license agreement governing the use of IBM software.

14. THANK YOU!Gireesh Punathil | gpunathi@in.ibm.com | @gireeshpunam | https://github.com/gireeshpunathil