A shadow over our clean, elegant code? Photo by Benjamin Bousquet Recently I got into a about how to decide a function is pure in JavaScript. The whole concept of pureness seems to be blurry in such a dynamic language. The following examples show we might need to redefine the term ‘pure function’, or — at least–be very careful when we decide on it. discussion What is a pure function If you are new to this term I recommend you first read some introduction. by and by are excellent choices. The Definition of “Pure Function” Alvin Alexander Master the JavaScript Interview: What is a Pure Function? Eric Elliott To sum up, a function is called pure if it satisfies the two conditions: The function returns exactly the same result every time it’s called with the same set of arguments. Evaluation of the function does not modify some state outside its scope nor has an observable interaction with the outside world besides returning a value. (No side effects.) Sometimes, a third condition is added: ‘Relies on no external mutable state.’ This, in fact, is redundant as such dependency on mutable variable would inevitably lead to breaking the first condition, too. Which of these is pure Here I wrote four example functions. Before proceeding, please review them and decide, on your own, which is pure and which is impure. Done? Great, let's compare. When I asked around, the vast majority answered that function is the only impure, functions , , and are pure. doubleB doubleA doubleC doubleD So let's go through the conditions. The latter one is oblivious; there are no side effects. The first one, is more interesting. When called with the same arguments all of them return the same value (using to support arrays): toEqual expect( doubleA(1) ).toEqual( doubleA(1) )expect( doubleB(1) ).toEqual( doubleB(1) )expect( doubleC(1) ).toEqual( doubleC(1) )expect( doubleD([1]) ).toEqual( doubleD([1]) ) Right? Right? Weeeell, written like this yes. However, what about this piece of code my friend with? Alexander replied doubleB(1) // -> 2 two = 3 doubleB(1) // -> 3 This is valid. I ran the function twice with the same argument and received different value. That makes it impure. No matter what happened in between. That made me think. If this is the proof, what about the others? Will they hold if I try hard enough? As you guess, no, they don't. In fact, I now say: None of the four functions is pure. Functions are first-class In JavaScript functions are first-class, meaning they are a value of a variable, that can be passed, returned, and–yes–reassigned. If I can change the variable , I can do the following: two doubleC(1) // -> 2 getTwo = function() { return 3 } doubleC(1) // -> 3 It is important to emphasise that this does not differ to what Alex did above. Just instead of holding a number, the variable holds a function. Map on array that was the name of one of my . These three are the core of data transformation in functional programming. So they should be safe to use in pure functions. “Map, filter, reduce. Repeat.” livecoding sessions As it turns out, in JavaScript nothing is set in stone. Or should I say in ? prototype doubleD([1]) // -> [2] Array.prototype.map = function() {return [3]} doubleD([1]) // -> [3] Wait. That is surely not allowed. This is wrong. It might be wrong, it might be stupid. The truth is, I just called the function twice with the same argument and received different values. . doubleD No matter what happened in between All we do is reassigning variables between the function calls. As we did before. Therefore, is not pure. doubleD Multiplication is pure However dynamic, in JavaScript one cannot override built-in operators like in . some languages Also, is a local variable living only in that function's scope. No way to change it from outside. n Or is it? No, it really is not possible. You must think low of JavaScript if you got your hopes up 😄 But I gave myself away when I wrote that none of the four functions is pure. There's another trick up my sleeve. While I cannot change the operation nor the argument after it's passed, I have a freedom of choosing what to pass. Numbers, strings, booleans, objects… Objects? What use can they have? A number multiplied by an object is, eh, is… like in `2 * {}`, is… (Go check it out in the console.) (As I did.) NaN. That doesn't help, though. If only there was a way to make the runtime convert the object to a number when multiplied. toString for numbers If an object appears in a string context, like concatenation with a string, the engine will run function of the object and use the result. If it's not implemented, it will fallback to known produced by method. toString '[object Object]' Object.prototype.toString While less used, JavaScript also calls method of an object when it expects a number (or a boolean, or a function). What's left is to make this function return different value each time it is invoked. valueOf var o = {valueOf: Math.random} doubleA(o) // -> 1.7709942335937932 doubleA(o) // -> 1.2600863386367704 Run this in JS Bin. Uff, yes. The function was called twice with the exactly same (by any comparing mean) argument, the second time it returned a different value than the first time. It is not pure. @@toPrimitive Symbol.toPrimitive_. As_ valueOf @@toPrimitive_, you still might want to_ Note: The previous version of this article used or, more verbose, Alexandre Morgaut pointed out, is sufficient and supported since the first version of JavaScript. If you don't know check it out . What is a pure function, again I know, I am being scrupulous, evil, and I use some dirty tricks. We use pure functions to gain confidence over the code. To be sure the code does what it should. Anytime, under any circumstances. if I decide to. Yes, that includes functions like . What if that variable ( in our case) is not supposed to be changed, it's a mathematical constant e, pi, or ? That should be pure. I want all of the four functions to be pure doubleB two phi I want to be able to trust built-in functions. What kind of programs can I create if I assume anything in or can change? Extremely basic ones; nobody would ever want to use them. Array.prototype Object.prototype As a result of this small, fun exercise I believe . Unfortunately, I see no way this could be limited only to technical terms. we need a new definition of what we consider a pure function in JavaScript In some way, it must take into account the intended use of the code. A function can be considered pure in one project and impure in another. And it is OK. As long as the program works. As long as the developers have confidence. Do you have an idea for the definition? How do you decide a function is pure? Is there something I missed? Did you learn something? Remarks There are ways to protect against of the tricks used above. some some Overwriting a free variable like or can be avoided by encapsulating the whole block into a function. Either using or modules: two getTwo IIFE var doubleB = (function () {var two = 2 return function (n) {return n * two}})() A better approach would be to use introduced in ES2015: const const two = 2const doubleB = (n) => n * two Preventing abuse of or is also possible, yet cumbersome. For example like this: valueOf @@toPrimitive function doubleA(n) {if (typeof n !== 'number') return NaN return n * 2} One could get around the trick with changing the only by avoiding such functions and falling back to loops. That is ugly, impractical, and potentially impossible. Abstracting these or using a library has drawbacks on its own. Array.prototype for (for … of) Don't forget that to make a function truly pure, one would need to combine all of those anti-tricks together. Imagine how doubleD, now that elegant, would look like, how long it would be, and how it would hurt the readability. 👏 . If you like this post, please don’t forget to give a below Every clap notification is a motivational boost for me. If you would like to learn more, I recently started a YouTube channel about JavaScript. I post new video every week, so consider subscribing. Be there from the beginning and help me get better. _JavaScript is my passion: I like to write JavaScript, I like to read JavaScript, and I like to talk JavaScript._www.youtube.com/c/robinpokorny Robin Pokorny on YouTube

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