Discussion

1. Eric Elliott

   This is a good start, but the philosophy of side-effect free programming runs much deeper, and forms the foundation of the functional programming paradigm. To help people understand the philosophical underpinnings, I wrote “The Dao of Immutability”:https://medium.com/javascript-scene/the-dao-of-immutability-9f91a70c88cd
2. Island

   Very informative article, david. I have touched up the unfinishedforEach function for the reader to continue readinghttp://gadgets-code.com/javascript-array-prototype-foreach-method-usage
• Philip Danic

  Nice follow-up, but I would really encourage you to useconsole.log() in such examples instead ofalert().
3. Island

   Alright, thanks for the advice :)
4. Radek

   Hi, thanks for the article. I hope you continue writing on the topic.

   I think the utility variables should not be an issue as long as they do not leak out from methods and variables are always initialized before usage. What bothers me more is the length variable which is global.
• David

  I’m curious as to why one would define alength variable within the for loop for the array iteration when one could just directly access the arraylength property, since in the example, thelength value is never modified.
• Andreas

  Because computing the length could be expensive. You would do it over and over instead of accessing a variable which is cheaper.
• David

  :( Javascript doesn’t cache that length computation behind the scenes I guess?
• Radek

  You are right – in this example we would get away with using thelength property. In case of other languages or data structures it is a good habit to calculate the iteration count beforehand.

  Btw. I misread the semicolons –length variable is not global.
5. Anna

   What about performance though? Regularfor loop seems to be more efficient thanforEach –http://jsperf.com/for-vs-foreach/37 .
6. Danik

   I’m confused. I thought it was considered bad practice to useforEach on an array?
7. balaphp

   What about this one and is there any side effects?

    var myArray = [1, 2, 3];for(index in myArray) {// ...}
• Drew

  the for construct doesn’t protect or define index in that case: this is implicitly creating var index, which then leaks. Just run that code in the console and check what “index” is afterwards. It’ll have a value, and probably not even one you’d expect.
• Adam

  usefor(index of myArray) instead ofin. It still has the side effect butof does not loop through all properties of the array.
8. Peter

   I found this article when I was looking for how to manage side-effects/side-causes in JavaScript a few months ago. Recently I have been experimenting with creating Intentions in JavaScript. They act like promises but build a description of what should be done that is only executed when explicitly called. It might be of interest for other people showing up here.https://github.com/CrowdHailer/intention.js

   e.g.

   var getPage = new HTTPAction(function (fetch) {  return fetch('/users.html').then(function (response) {    return response.text()  })})// No externam call is made untill the action is rungetPage.run(window.fetch).then(function (body) {  console.log(body)})
9. Adam

   to David W. using the anonymous icon for guest user is deceiving of an experienced user. ;)
10. Worker11811

    In my experience usingforEach almost always leads to side effects…