Monthly Archives: June 2017

Lodash Performance Sucks!

To continue my Functional Programming Sucks series of posts I will have a closer look at reduce().

I complained with Lodash (and Underscore) for different reasons. One complaint was performance, but I just read the code and presumed it was going to be slow without measuring. Then I complained with the performance of Functional Programming in general.

I thought it would be interesting to “improve” the Functional code with Lodash functions, and to my surprise (I admit I was both wrong and surprised) I found Lodash made it faster! After reading a little more about it I discovered this is a well known fact.

So, here are four different implementations of a function that checks if the elements (numbers) in an array are ordered (cnt is incremented if the array is sorted, such was the original problem).

```// Standard reduce()
this.test = function(p) {
if ( false !== p.reduce(function(acc,val) {
if ( false === acc || val < acc ) return false;
return val;
}, -1)) cnt++;
};

// Lodash reduce(), and some other Lodash waste
this.test = function(p) {
if ( false !== LO.reduce(p,function(acc,val) {
if ( false === acc || val < acc ) return false;
//      if ( !LO.isNumber(acc) || val < acc ) return false;
return val;
}, -1)) cnt++;
};

// My own 4 minute to implement simpleReduce(), see below
this.test = function(p) {
if ( false !== simpleReduce(p,function(acc,val) {
if ( false === acc || val < acc ) return false;
return val;
}, -1)) cnt++;
};

// A simple imperative version
this.test = function(p) {
var i;
for ( i=1 ; i < p.length ; i++ ) {
if ( p[i] < p[i-1] ) return;
}
cnt++;
};

// my own implementation reduce()
function simpleReduce(array, func, initval) {
var i;
var v = initval;
for ( i=0 ; i<array.length ; i++ ) {
v = func(v, array[i]);
}
return v;
}
```

The interesting thing here is that the standard library reduce() is the slowest.
However, my simpleReduce is faster than Lodash reduce().

 (seconds) reduce() Std Lib Lodash Simple Imperative Raspberry Pi v1 (ARMv6 @ 700) 21 13 9.3 4.8 MacBook Air (Core i5 @ 1400) 0.46 0.23 0.19 0.16

Conclusion
The conclusion is that from a performance perspective Functional Programming sucks. Lodash sucks too, but a little bit less so than the standard library (however, if you decorate all your code with isEmpty, isString, isNumber and that crap it will get worse).

That said, the generic nature of Lodash comes at a cost. The most simpleReduce() imaginable outperforms Lodash. As I see it, this leaves Lodash in a pretty bad (or small) place:

• Compared to the standard library it is an extra dependency with limited performance benefits
• The generic nature of Lodash comes at both a performance cost and it allows for sloppy coding
• A hand written reduce() outperforms Lodash and is a good excercise for anyone to write. I expect this is quite true also for other functions like take or takeRight.
• For best performance, avoid Functional Programming (and in this case the imperative version is arguably more readable than the FP reduce() versions)

Whats up with the Standard Library???
JavaScript is a scripted language (interpreted with a JIT compiler) that has a standard library written in C++. How can anything written in JavaScript execute faster than anything in the standard library that does the same thing?

First, kudos to the JIT designers! Amazing job! Perhaps the standard library people can learn from you?

I can imagine the standard library functions are doing some tests or validations that are somehow required by the standard, and that a faster and less strict version of reduce() would possibly break existing code (although this sounds far fetched).

I can (almost not) imagine that there is a cost of going from JS to Native and back to JS: that function calls to native code comes with overhead. Like going from user space to kernel space. It sounds strange.

I have read that there are optimizations techniques applied to Lodash (like lazy evaluation), but I certainly didn’t do anything like that in my simpleReduce().

For Node.js optimizing the standard library truly would make sense. In the standard library native code of a single-threaded server application every cycle counts.

UPDATE: I tried replacing parts of the above code: 1) the lambda function that is passed to reduce(), 2) the imperative version, with native code. That is, I wrote C++ code for V8 and used it instead of JavaScript code. In both cases this was slower! Obviously there is some overhead in going between native and JavaScript JIT, and for rather small functions this overhead makes C++ “slower” than JavaScript. My idea was to write a C++ reduce() function but I think the two functions I wrote are enough to show what is happening here. Conclusion: don’t write small native C++ functions for performance, and for maximum performance it can be worth to rewrite the standard library in JavaScript (although this is insane to do)!

All FP-sucks related articles
Functional Programming Sucks)
Underscore.js sucks! Lodash sucks!
Functional Programming Sucks! (it is slow)
Lodash Performance Sucks! (this one)

Functional Programming Sucks! (it is slow)

Update 2018-08-26: I got into a discussion about never using loops. The code that was discussed was checking if a word is a palindrome (the same word if read backwards). I wrote some simple performance tests and you can draw your own conclusions here.

Update 2017-12-05: I added a new test in the end that came from real code.
It is both true that functional code is slower and that Node.js v8 is tightening the gap.

Update 2017-07-17: Below i present numbers showing that functional code is slower than imperative code. It seems this has changed with newer versions of Node.js: functional code has not turned faster but imperative code has become slower. You can read a little more about it in the comments. I will look more into this. Keep in mind that the below findings may be more accurate for Node.js v4-6 than for v8.

Functional programming is very popular with contemporary JavaScript programmers. As I have written before, Functional programming sucks and functional libraries for JavaScript also suck.

In this post I will explain more why Functional Programming sucks. I will start with the conclusion. Read on as long as you want more details.

Functional Programming practices are bad for performance
It is very popular to feed lamda-functions to map(), reduce(), filter() and others. If you do this carelessly the performance loss is significant.

It is also popular to work with immutable data. That is, you avoid functions that change (mutate) current state (side effects) and instead you produce a new state (a pure function). This puts a lot of pressure on the garbage collector and it can destroy performance.

The Benchmark Problem
Sometimes I entertain myself solving problems on Hackerrank.com. I particularly like the mathematical challenges in the Project Euler section (the Project Euler is also an independent organisation – HackerRank uses the challenges in Project Euler to create programming challenges).

This article refers to Project Euler 32. I will not go into details, but the solution is basically:

1. Generate all permutations of the numbers 1,2,3,4,5,6,7,8,9 (there are 9! of them)
2. For each permutation, check if it is “good” (very few are)
3. Print the sum of the good instances

The first two steps give good benchmark problems. I have made different implementations of (1) and (2) and then compared the results.

Benchmark Results
I have three different permutation generators (all recursive functions):

1. Pure function, immutable data (it may not be strictly pure)
2. Function that mutates its own internal state, but not its input
3. Function that mutates shared data (no allocation/garbace collection)

I also have three different test functions:

1. Tests the orginal Project Euler problem
2. Simplified test using reduce() and lamda function
3. Simplified test implemented a standard loop

I benchmarked on two different systems using Node.js version 6. I have written elsewhere that Node.js performance on Raspberry Pi sucks.

 (seconds) Project Euler Test Simplified Test Test Function: Functional Imperative Permutation Gen: Pure Semi Shared Shared Shared Pure Raspberry Pi v1 (ARMv6 @ 700) 69 23 7.4 21 3.7 62 MacBook Air (Core i5 @ 1400) 0.77 0.29 0.13 0.40 0.11 0.74

Comparing columns 1-2-3 shows the performance of different generators (for Project Euler test)
Comparing columns 4-5 shows the performance of two different test functions (using fast generator)
Comparing columns 5-6 shows the performance of two different generators (for fast simple test)

This shows that the benefit of using shared/mutable data (not running the garbage collector) instead of immutable data is 5x performance on the Intel CPU and even more on the ARM. Also, the cost of using reduce() with a lamda function is more than 3x overall performance on the Intel CPU, and even more on the ARM.

For both the test function and permutation generation, making any of them functional-slow significantly slows down the entire program.

The conclusion of this is that unless you are quite sure your code will never be performance critical you should avoid functional programming practices. It is a lot easier to write imperative code than to later scale out your architecture when your code does not perform.

However, the pure immutable implementation of the permutation generator is arguably much simpler than the iterative (faster) counterpart. When you look at the code you may decide that the performance penalty is acceptable to you. When it comes to the reduce() with a lamda function, I think the imperative version is easier to read (and much faster).

Please notice that if your code consists of nice testable, replaceble parts without side effects you can optimize later on. The functional principles are more valuable at a higher level. If you define your functions in a way that they behave like nice FP functions it does not matter if they are implemented using imperative principles (for performance).

Generating Permutations
I used the following simple method for generating permutations. I start with two arrays and I send them to my permute-function:

```  head = [];
tail = [1,2,3,4];

```

My permute-function checks if tail is empty, and then: test/evalute head.
Otherwise it generates 4 (one for each element in tail) new sets of head and tail:

```  permute( [1] , [2,3,4] )
permute( [2] , [1,3,4] )
permute( [3] , [1,2,4] )
permute( [4] , [1,2,3] )
```

The difference in implementation is:

• Pure version generates all the above 8 arrays as new arrays using standard array functions
• Semi pure version generates its own 2 arrays (head and tail) and then uses a standard loop to change the values of the arrays between the (recursive) calls to permute.
• Shared version simply creates a single head-array and 9 tail-arrays (one for each recursion step) up front. It then reuses these arrays throughout the 9! iterations. (It is not global variables, they are hidden and private to the permutation generator)

The simplified test
The simplified test checks if the array is sorted: [1,2,3,4]. Of all permutations, there is always exactly one that is sorted. It is a simple test to implement (especially with a loop).

```// These functions are part of a "test-class" starting like:
function testorder1() {
var cnt = 0;

// Functional test
this.test = function(p) {
if ( false !== p.reduce(function(acc,val) {
if ( false === acc || val < acc ) return false;
return val;
}, -1)) cnt++;
};

// Iterative test (much faster)
this.test = function(p) {
var i;
for ( i=1 ; i<p.length ; i++ ) {
if ( p[i] < p[i-1] ) return;
}
cnt++;
};
```

I tried to optimise the functional reduce() version by breaking out a named function. That did not help. I also tried to let the function always return the same type (now it returns false OR a number) but that also made no difference at all.

All the code
For those who want to run this themselves or compare the permutation functions here is the entire program.

As mentioned above, the slowest (immutable data) permutation function is a lot smaller and easier to understand then the fastest (shared data) implementation.

```
'use strict';

// UTILITIES

function arrayToNum(p, s, e) {
var r = 0;
var m = 1;
var i;
for ( i=e-1 ; s<=i ; i-- ) {
r += m * p[i];
m *= 10;
}
return r;
}

function arrayWithZeros(n) {
var i;
var a = new Array(n);
for ( i=0 ; i<a.length ; i++ ) a[i] = 0;
return a;
}

// PERMUTATION ENGINES

function permutations0(n, callback) {
}

// IMMUTABLE (SLOWEST)

function permutations1(n, callback) {
var i;
var numbers = [];
for ( i=1 ; i<=n ; i++ ) numbers.push(i);
permute1([],numbers,callback);
}

if ( 0 === tail.length ) {
return;
}

tail.forEach(function(t, i, a) {
a.slice(0,i).concat(a.slice(i+1)),
callback);

});
}

// MUTATES ITS OWN DATA, BUT NOT ITS ARGUMENTS

function permutations2(n, callback) {
var i;
var numbers = [];
for ( i=1 ; i<=n ; i++ ) numbers.push(i);
permute2([],numbers,callback);
}

if ( 0 === tail.length ) {
return;
}
var t2 = tail.slice(1);
var i  = 0;
var tmp;

while (true) {
permute2(h2, t2, callback);
if ( i === t2.length ) return;
tmp   = h2[0];
h2[0] = t2[i];
t2[i] = tmp;
i++;
}
}

// MUTATES ALL DATA (INTERNALLY) (FASTEST)

function permutations3(n, callback) {
var i;
var tails = new Array(n+1);

for ( i=1 ; i<=n ; i++ ) {
tails[i] = arrayWithZeros(i);
}

for ( i=1 ; i<=n ; i++ ) {
tails[n][i-1] = i;
}

function permute3(x) {
var j;
var tail_this;
var tail_next;
var tmp;
if ( 0 === x ) {
return;
}
tail_this = tails[x];
tail_next = tails[x-1];

for ( j=1 ; j<x ; j++ ) {
tail_next[j-1] = tail_this[j];
}

j=0;
while ( true ) {
permute3(x-1);

j++;
if ( j === x ) return;

tail_next[j-1] = tmp;
}
}

permute3(n);
}

// TEST FUNCTIONS

function testprint() {
this.test = function(p) {
console.log(JSON.stringify(p));
};

this.done = function() {
return 'Done';
};
}

// CHECKS IF PERMUTATION IS ORDERED - FUNCTIONAL (SLOWEST)

function testorder1() {
var cnt = 0;

this.test = function(p) {
if ( false !== p.reduce(function(acc,val) {
if ( false === acc || val < acc ) return false;
return val;
}, -1)) cnt++;
};

this.done = function() {
return cnt;
};
}

// CHECKS IF PERMUTATION IS ORDERED - IMPERATIVE (FASTEST)

function testorder2() {
var cnt = 0;

this.test = function(p) {
var i;
for ( i=1 ; i<p.length ; i++ ) {
if ( p[i] < p[i-1] ) return;
}
cnt++;
};

this.done = function() {
return cnt;
};
}

// TEST FUNCTION FOR PROJECT EULER 32

function testeuler() {
var sums = {};

this.test = function(p) {
var w1, w2, w;
var m1, m2, mx;
w =  Math.floor(p.length/2);
w1 = 1;
w2 = p.length - w - w1;

while ( w1 <= w2 ) {
m1 = arrayToNum(p,     0, w1      );
m2 = arrayToNum(p,    w1, w1+w2   );
mx = arrayToNum(p, w1+w2, p.length);

if ( m1 < m2 && m1 * m2 === mx ) {
sums['' + mx] = true;
}

w1++;
w2--;
}
};

this.done = function() {
var i;
var r = 0;
for ( i in sums ) {
r += +i;
}
return r;
};
}

// MAIN PROGRAM BELOW

function processData(input, parg, targ) {
var r;

var test = null;
var perm = null;

switch ( parg ) {
case '0':
perm = permutations0;
break;
case '1':
perm = permutations1;
break;
case '2':
perm = permutations2;
break;
case '3':
perm = permutations3;
break;
}

switch ( targ ) {
case 'E':
test = new testeuler;
break;
case 'O1':
test = new testorder1;
break;
case 'O2':
test = new testorder2;
break;
case 'P':
test = new testprint();
break;
}

r = perm(+input, test.test);
console.log(test.done());
}

function main() {
var input = '';
var parg = '1';
var targ = 'E';
var i;

for ( i=2 ; i<process.argv.length ; i++ ) {
switch ( process.argv[i] ) {
case '0':
case '1':
case '2':
case '3':
parg = process.argv[i];
break;
case 'E':
case 'O1':
case 'O2':
case 'P':
targ = process.argv[i];
break;
}
}

process.stdin.resume();
process.stdin.setEncoding('ascii');
process.stdin.on('data', function (s) {
input += s;
});

process.stdin.on('end', function () {
processData(input, parg, targ);
});
}

main();
```

This is how I run the code (use a lower value than 9 to have fewer than 9! permutations)

```### Project Euler Test: 3 different permutation generators ###
\$ echo 9 | time node projecteuler32.js 3 E
45228
8.95user ...
b\$ echo 9 | time node projecteuler32.js 2 E
45228
25.03user ...
\$ echo 9 | time node projecteuler32.js 1 E
45228
70.34user ...

### Simple check-order test, two different versions. Fastest permutations.
b\$ echo 9 | time node projecteuler32.js 3 O1
1
23.71user ...
\$ echo 9 | time node projecteuler32.js 3 O2
1
4.72user ...
```

(the timings here may not exactly match the above figures)

Update 2017-12-05
Admittedly, I sometimes find map(), filter() handy and I try to use them when it makes code more clear. I came to a situation where I want to split a list in two lists (one list with valid objects and one with invalid). This is a simple if/else with a push() in each. Or it would be two calls to filter(). Then it turned out that I wanted to split the valid objects into two lists: good and ugly. The slightly simplified code is:

```function goodBadUgly_1(list) {
var i, c;
var ret = {
good : [],
ugly : []
}
for ( i=0 ; i<list.length ; i++ ) {
c = list[i];
if ( !validateItem(c) )
else if ( uglyItem(c) )
ret.ugly.push(c);
else
ret.good.push(c);
}
return ret;
}

return {
good : list.filter(function(c) {
return validateItem(c) && !uglyItem(c);
}),
return !validateItem(c);
}),
ugly : list.filter(function(c) {
return  validateItem(c) && uglyItem(c);
})
};
}
```

On my not too powerful x64 CPU, and a list of about 1000 items the non-FP version took 6ms and the FP version took 16ms (second run, to allow the JIT to do its job). This was with Node 8.9.1. For Node 6.11.3 the FP version was slower but the non-FP version was almost same speed (quite consistent with my comment in the beginning from 2017-07-17).

You may think that of course the FP code is slower: it calls validateItem twice (always) and uglyItem twice for all valid items. Yes, that is true, and that is also my point! When you do FP you avoid (storing intermediate results in) variables. This results in extra work being done a lot of the time. How would YOU implement this in FP style?

This is 10 ms: does it matter? Well, first it is only 1000 objects.

If you do this in a Web GUI when a user clicks a button, the user will wait 10ms longer for everything to be updated. 10ms is not a lot. But if this multiplies (because you have a longer list) or adds up (because you are doing other things in a slower-than-necessary way) the UX will suffer.

If you do this server side, 10ms is a lot. In Node.js you have just 1 thread. So this overhead is 1% of all available performance each second. If you get 10 requests per second 10% CPU is wasted only because you prefer FP style.

This is one of those cases when FP has the same computational complexity, but its kind of a constant factor slower. Sometimes it can be even worse.

Update 2018-08-26: Palindrome test
Someone suggested to me to check whether a word is a palindrome using "functional programming" and a reverse() higher order function. The alternatives are:

```const isPalindromeStd = (str) => {
let a=0;
let b=str.length-1;
while ( a<b ) {
if ( str[a] !== str[b] ) return false;
a++; b--;
}
return true;
};

const reverseStr = (str) => {
return Array.from(str).reverse().join('');
};

const isPalindromeRev = (str) =>  {
return str === reverseStr(str);
};
```

Admittedly, the loop-less version has some simplistic appeal. Both of them are equally testable and they are used identically, thus allowing the rest of your program to be equally "functional". When it comes to performance, when testing on a file with common passwords the Std is 20x faster than the Rev implementation. In fact, for practical purposes Std is on average O(1) while Rev is O(n). You can see for yourself here.

All FP-sucks related articles
Functional Programming Sucks)
Underscore.js sucks! Lodash sucks!
Functional Programming Sucks! (it is slow) (this one)
Lodash Performance Sucks!

Underscore.js sucks! Lodash sucks!

In a world of functional programming hype there are two very popular JavaScript frameworks: underscore.js and Lodash. Dont use them! It is a horrible idea. They suck just like functional programming sucks!

They make claims like:

• Lodash: A modern JavaScript utility library delivering […] performance
• Underscore: JavaScript library that provides a whole mess of useful functional programming helpers.

The road to hell is sided by good intentions. This is how it goes.

1. Sloppy data types
There are many good things about JavaScript. The sloppy dynamic typing is perhaps not one of them. The following are for example true:

• ’27’ == 27
• undefined == null
• 0 == ”
• ‘object’ === typeof null

Now that I consider myself an experienced programmer I find it quite convenient to not need to be explicit about data types. But I dont mix and change types! If a variable is a number from the beginning it keeps being a number. I carefully pick types: Objects, Arrays, Number, String and stick to that type (for a variable or property). Occationally – mostly for return variables – I break the rule.

Lodash and Underscore is about allowing yourself to be sloppy:

• Dont know if its an object or an array: use map, foreach, filter, reduce and many more
• Dont know if it is empty (or what being empty even means): use isEmpty
• Dont know if it is String Object or a String primitive or something else: use isString

If you dont know what it is is you already have a much bigger problem than how to do something with it.
If you mix String Objects and String primitives AND other things, and you want to know if it is any kind of string you are doing something wrong.

So Step 1 with Lodash and Underscore is that you

2. Allow sloppy and inconsistent typing
4. Your code is now impossible to maintain or extend without lodash or underscore

2. Performance!
My experience after many years in software development is that when an application is not well received by the users it is very often because of (bad) performance. And bad performance causes weird, hard to reproduce, bugs and instability as well.

An important type of optimization that the JIT can do relies on the runtime generating classes with strict types for your objects (it guesses and learns the types as the program runs). If you allow a property to assume values of different types you are likely to destroy this optimization.

Lets look at the little function isEmpty.

```/** Underscore **/
_.isEmpty = function(obj) {
if (obj == null) return true;
if (isArrayLike(obj) && (_.isArray(obj) || _.isString(obj) || _.isArguments(obj))) return obj.length === 0;
return _.keys(obj).length === 0;
};

/** Lodash **/
function isEmpty(value) {
if (isArrayLike(value) &&
(isArray(value) || isString(value) ||
isFunction(value.splice) || isArguments(value))) {
return !value.length;
}
return !nativeKeys(value).length;
}
```

If you KNOW the datatype you can just do:

```/** String **/
if ( 0 === s.length )

/** String that may be null **/
if ( null === s || 0 === s.length )

/** Array **/
if ( 0 === a.length )

/** Object **/
function objectIsEmpty(o) {
for ( x in o ) return false;
return true;
}
```

(you may want to check o.hasOwnProperty(x) depending on what you actually want – but if you dont know what you want using Lodash or Underscore will produce equally unexpected results as my code)

The worst thing with the Underscore and Loadash implementations are the last lines:

```    return _.keys(obj).length === 0;
return !nativeKeys(value).length;
```

Unless the JIT compiler and runtime is very smart those two will produce an entire array of strings on the heap – just to check if the array length is 0! Even though this in most practical cases will have an acceptable overhead it can get very expensive.

This IS the beauty of FP generic programming. Focus on WHAT, not HOW, and any little innocent check (like isEmpty) can turn horribly expensive.

However, to be honest, I took some plain functional code and replaced plain JavaScript with Lodash functions (forEach, reduce, isNumber, isEmpty and a few more) and the code actually got faster! Still much slower than imperative code, but slightly faster than not using Lodash.

3. Complexity and Optimization
Now that you have added an extra dependency, made your data objects sloppy, made your application harder for the JIT to optimize, perhaps your application is not as quick as you need it to be. If you are writing a front end you are probably quite fine. But if you are coding a Node.js backend, performance matters a lot more and waste is more unacceptable. If you are really unlucky these sloppy types give you hard to find bugs and your backend is not completely stable and reliable.

What do you do? Common practices in the business could be things like:

• Minification/uglification
• Scale out service architecture
• Failover technology
• Spend time optimizing code and modules

This is how little sloppyness, laziness and convenience when making initial decisions about your architecture later can cause you huge problems and costs.

Of course, just including lodash and using isEmpty() in a few places is not going to do much (if any) harm.
But finding lodash or underscore generally preferable to not using them is one kind of low quality thinking that causes software to be bad.

Conclusion
Be explicit, careful, consistent and smart about the data types you use.
Be restrictive with libraries and frameworks that introduce overhead and hide relevant details.

Use the standard library. However, you can find that for example the Array functions of Lodash outperform the standard library equivalents. That may not be true in the future (and I really wonder how it can happen at all).

All FP-sucks related articles
Functional Programming Sucks)
Underscore.js sucks! Lodash sucks! (this one)
Functional Programming Sucks! (it is slow)
Lodash Performance Sucks!

Functional Programming Sucks*

(*like everything else that is mindlessly applied the wrong way to solve the wrong problems)

Functional Programming Rocks!
You can do amazing things with Haskell. The historical importance of Lisp is huge. Math, computer science, algorithms and engineering can come together beautifully with functional programming. Writing small reusable, testable functions with no side effects is a great thing to do in any language – but more than anywhere else those virtues are emphasized in functional programming. I love first class functions!

But…

The unfortunate JavaScript Hype
As JavaScript made map, filter and reduce part of the standard those have become the foremost frontier of functional programming. Being quite serious, many people argue that if you replace your for-loops with map, filter, reduce and forEach you have achieved something significant when it comes to readability. What do you think?

```// non-functional smelly loop
redFruits = [];
for ( f in fruits ) {
if ( 'red' === fruits[f].color ) redFruits.push(fruits[f]);
}

// fantastic functional alternative
greenFruits = fruits.filter(function(f) {
return 'green' === f.color;
});
```

As a bonus with functional, you can use lamda-functions… that is functions, with no name, that can not be reused. You can even use arrow-functions if you think this is too easy a concept and you think the code gets more readable by omitting the confusing word function.

While there are a lot of balanced articles about using functional ideas in JavaScript, for a lot of people eliminating for-loops at any cost seems to be functional enough.

The argument here is that your code should be declarative, it should express intention rather than instructions. In the case above with the fruit-filter it does make some sense. But if that argument should make any sense, it requires that the programmer does not abuse map, filter, forEach or reduce to eliminate a for-loop just for the sake of it. I have seen things like:

```// functional
applePrice = fruits[Object.keys(fruits).filter(function(f) {
return 'apple' === fruits[f].name;
})[0]].price;

// when this would work
for ( f in fruits ) {
if ( 'apple' === fruits[f].name ) {
applePrice = fruits[f].price;
break;
}
}
```

I have several objections with the functional version. The Object.keys thing is hardly pleasant to the eye. It is false marketing to use filter: the intention is find or search, not filter. So you still need to read the details (just as with the for-loop), you are just first fooled into thinking its a filter (and chaining is very popular, so then you have no function names and no variable names). But perhaps the worst problem is the lack of error handling. Functional code is not meant to have side effects, and error handling is exactly that. If ‘apple’ is not found you get an ugly Error. You can of course try/catch, or make temporary array variable apples and check that its length is one, but people who prefer functional style usually dont do it.

I understand your objection: people can write crappy code with any language an paradigm and just becuase I have seen bad applications of filter does not mean there is anything wrong with filter or FP. Of course. The bad thing is recommending people to use it like a silver bullet. The bad thing is that good FP is actually difficult and junior programmers will get it wrong trying to be fashionable.

Here is another example to show I am not inventing this rant.

Functional is preferable-Hype
Another favorite example of this functional hype is from rosettacode. Look at this amazing collection of implementations of a simple algorithm: the Luhn Algorithm. I suggest:

1. Read the introduction so you get some idea
2. Look at the C-implementation: imperative and simple. Testable and no side effects.
3. Look at the functional implementations: C++11, Common Lisp, Haskell, JavaScript (ES5.1 version), PicoLisp, Python, Rust, Scheme

Update: this is what the Luhn Algoritm looked like when this article was written: (link to Rosetta Code).

Look at Scala: there are two versions, a Functional Style and an Imperative style. The people att IOCCC would be jealous with this shit!

I can only come to one conclusion: the emperor is naked.

I mean, if you code in PicoLisp, for fun or for a good reason, and you have to implement the Luhn algorith, please do so! But to say “recommended” about the functional Scala code or to think that the C++11 code is in anyway more reasonable than the original C-code… it makes no sense. No real systems programmers will choose Rust over C based on this. And Python – a language known for its clarity… such a sad example.

Trendy fashionable “functional programmers” suck
So, the problem here is not primarily with Function Programming itself or the code that guru coders write with Functional Programming. The problem is that people with far too little theoretical background, training and experience follow the hype and the result is ugly.

Functional Programming Sucks (at)
There are some things that Functional Programming is not very good at: State and Side Effects. This is by design. From a FP perspective (and a general perspective as well) State and Side Effects are nasty and problematic: they should be avoided and when they cant be avoided they need special attention (like Monads).

The problem here is that JavaScript is mostly a programming language for the web. A simple (modern, single page) web application:

1. Loads data from a server
2. Presents data to the user
3. Lets the user update the data
4. Sends the data back to the server

This is (almost) all about state and side effects! You may have some functions for validation, filtering, sorting and calculations on the client, and those benefit from Functional Programming ideas. But your core enterprise is about state, side effects and error handling! Functional Programming is so unsuitable for this that a simple web page just can’t be written functionally, unless you start breaking rules. And this is of course what programmers end up doing because in the end of the day they have a real application to build for real users.

The difficult thing is to break the right rules in the right way at the right place for the right reason. This is architecture – mixing concepts, and designing how your application lives, its breaths and heartbeats. Architecture is difficult, and it is not made easier relying on unsuitable silver bullets.

Functional Reactive Programming
There is one thing that is even more hyped and even more theoretic than Functional Programming, and that is Functional Reactive Programming.

But it makes sense (as I understand it). It is not about taking Functional Programming one step further but about putting it in context. You have data that changes (signals, events, behaviours, whatever). That data can be pipelined with high quality through FP logic to produce the content of your GUI or the requests to the server.

Keep the functional parts functional, and let other parts of your application just deal with I/O, GUI and state. Dividing your code into separate modules with clear responsibilites and clear interactions have always been a good idea.

Performance
My experience is that when a real world application is not well received by the users a lot of the time its because performance sucks. When performance is bad usability is also bad, and stability gets bad (especially on the server side).

Functional programming is typically bad for performance.

• anonymous functions can often not be JIT compiled and will never be optimized
• recursion is nice, but rarely faster than a loop
• the chaining concept creates arrays and arrays and arrays just for throwing away, which is fun for the garbage collector, but it is not fast
• the immutable object concept is about generating new RO copies of object instead of changing objects, which is expensive and wasteful

Perhaps worse, since functional programming and proper use of and map(), filter(), reduce() and friends is hard (easy things get difficult) not so experienced programmers end up writing implementations with unnecessary computational complexity ( O(n) turns into O(n^2) ). It is not funny – you cant afford that for most anything that goes to production.

Agile, refactoring and generic code
It is HARD to design code properly from the beginning! Good objects, classes, functions, modules, packages, namings, dependency trees and architecture dont come for free! Agile and Refactoring are about accepting that the design will not be optimal the first time, thus we should not bother too much about it, but rather fix the code when we have learnt more about the problem and our code starts getting (too) ugly.

A strong argument for FP is that it is highly generic – which is true. But until a programmer has spent much time with her domain and problem she will not know what things can and should be made generic. Making things too generic is called overengineering, and it is perhaps the worst sickness in our industry.

I usually:

• allow myself some copy-paste until I see what code really gets repeated
• make my code as specific as possible, unless I see an obvious generalisation or the actual need for generalisation emerges
• dont worry too much about global variables in the beginning (after a while there will be a natural place for them or for what they represent)
• allow quite long functions until I see what parts of them actually do something meaningful on its own
• code quite defensively with lots of error handling (it usually pays of quite quickly)

This works for getting quick practical results. Later, during refactoring, when the code base has grown, and when I have learnt more about the domain, I can break out pieces of critical code that creates nice generic functions. But thinking FP first – no way!

All FP-sucks related articles
Functional Programming Sucks (this one)
Underscore.js sucks! Lodash sucks!
Functional Programming Sucks! (it is slow)
Lodash Performance Sucks!

Update 2018-01-14: I also wrote Programming paradigms rock and suck! for putting FP and other paradigms in context.
Update 2018-12-15: I wrote Lambda Functions considered Harmful.